postgres: Use tzdata from PG sources

[mmlb]

What kind of change does this PR introduce?

Feature

What is the current behavior?

PG is built against nixpkgs' tzdata, which is stale and will remain so for a bit still.

What is the new behavior?

PG is built to use its own embedded tzdata which is up to date.

[brainrake]

Buys us until next tz change to get rolling updates going.

[mmlb]

Result from quick build/test:

[16:38:51]-[~/p/g/s/p/src]-[manny@zennix]
                                                                                                               ../start-server.fish
                                                                                                                                    The files belonging to this database system will be owned by user "manny".
This user must also own the server process.

The database cluster will be initialized with this locale configuration:
  locale provider:   builtin
  default collation: C.UTF-8
  LC_COLLATE:  C.UTF-8
  LC_CTYPE:    C.UTF-8
  LC_MESSAGES: C.UTF-8
  LC_MONETARY: C.UTF-8
  LC_NUMERIC:  C.UTF-8
  LC_TIME:     C.UTF-8
The default database encoding has accordingly been set to "UTF8".
The default text search configuration will be set to "english".

Data page checksums are enabled.

fixing permissions on existing directory /home/manny/projects/github.com/supabase/postgres/src/.working/pgtest/data ... ok
creating subdirectories ... ok
selecting dynamic shared memory implementation ... posix
selecting default "max_connections" ... 100
selecting default "shared_buffers" ... 128MB
selecting default time zone ... America/New_York
creating configuration files ... ok
running bootstrap script ... ok
performing post-bootstrap initialization ... ok
syncing data to disk ... ok

initdb: warning: enabling "trust" authentication for local connections
initdb: hint: You can change this by editing pg_hba.conf or using the option -A, or --auth-local and --auth-host, the next time you run initdb.

Success. You can now start the database server using:

    /home/manny/projects/github.com/supabase/postgres/src/result/bin/pg_ctl -D /home/manny/projects/github.com/supabase/postgres/src/.working/pgtest/data -l logfile start

 '--prefix=/nix/store/2vmz8a0qlqzbz1nhfa1zmbjfa1lfvlzh-postgresql-17.11' '--with-openssl' '--with-libxml' '--with-icu' '--sysconfdir=/etc' '--libdir=$(lib)/lib' '--enable-debug' '' '--with-ossp-uuid' '--with-lz4' '--with-zstd' '--with-gssapi' '--with-pam' 'build_alias=' 'host_alias=' 'target_alias=' 'CC=gcc' 'CXX=g++' 'PKG_CONFIG=pkg-config' 'PKG_CONFIG_PATH=/nix/store/5qzr2w5n5gcgh8gp3mi8sdfh6ryypfqj-libxcrypt-4.5.2/lib/pkgconfig:/nix/store/bc1x5zsl9wyqj35yjd9c3f2gf1b4r7qv-libxslt-1.1.45-dev/lib/pkgconfig:/nix/store/srnvjrr38j7xmpcmjpndd6mw31lsfjrl-zlib-1.3.1-dev/lib/pkgconfig:/nix/store/inys8r1mg4mwlxnrwa3wqga3xmp1rmvi-readline-8.3p1-dev/lib/pkgconfig:/nix/store/lrx5q7fas4bldb5m342liwyy693sas0g-ncurses-6.5-dev/lib/pkgconfig:/nix/store/2ivy0r8ab3bnps5957vfrxcjfcgad661-openssl-3.6.0-dev/lib/pkgconfig:/nix/store/5nl2zr46dic7mxcw6cvbq85wa8c2gngh-libxml2-2.15.1-dev/lib/pkgconfig:/nix/store/h5hfqhzcqszmsacj26hh3fngwcal04zs-icu4c-75.1-dev/lib/pkgconfig:/nix/store/sl10j863s32x7dv5vw6wfx1nh22f3xh2-lz4-1.10.0-dev/lib/pkgconfig:/nix/store/di94mxmhhk390zzzv2gvavfwkgww7q6h-zstd-1.5.7-dev/lib/pkgconfig:/nix/store/20dygnb42nwy7mbilwgql2lzz8h69pqz-krb5-1.22.1-dev/lib/pkgconfig:/nix/store/adcxk93kpci5li5plbfka0sciqwq9jdf-linux-pam-1.7.1/lib/pkgconfig:/nix/store/sadhrdhfxqix4skq8phrvhw1qhhyipq3-libossp-uuid-1.6.2/lib/pkgconfig'
waiting for server to start.... done
server started
                                    version
-------------------------------------------------------------------------------
 PostgreSQL 17.11 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 15.2.0, 64-bit
(1 row)

psql (17.11)
Type "help" for help.

postgres=# select '2026-12-15 18:00+00'::timestamptz at time zone 'America/Edmonton';
      timezone
---------------------
 2026-12-15 12:00:00
(1 row)

postgres=#
\q
waiting for server to shut down.... done
server stopped

[blacksmith-sh]

Blacksmith runners detected OOM events on the following jobs:

Job	Details
Package Dependency Analysis	View Job

[mmlb]

GHA wasn't happy with the nix-eval step and I couldn't quite debug it from logs available. I had some not-quite-working code to split the run nix-eval-jobs, process nix-eval-jobs output in github-matrix into 2 parts that I cleaned up/finished and now things are ok. Will keep it in.