Previously, Postgres was built in 'tmp_install/build', and installed
into 'tmp_install'. In the CI, the 'build' directory was included in
the final neon.tar.zst artifact that includes all the necessary
binaries. That was unnecessary, the intermediate build results are not
needed, only the final binaries. Separate the build directory so that
the Postgres build happens in 'build', and it is installed into
'tmp_install'. That makes the final neon.tar.zst artifact smaller.
The changes to the python tests are needed to find the 'pg_regress'
binary in the installation directory. Previously, they would use the
'pg_regress' binary from the build directory, not the final
installation location.
Simplifies the workflow. Makes the overall build a little faster, as
the build_postgres step doesn't need to upload the pg.tgz artifact,
and the build_neon step doesn't need to download it again.
This effectively reverts commit a490f64a68. That commit changed the
workflow so that the Postgres binaries were not included in the
neon.tgz artifact. With this commit, the pg.tgz artifact is gone, and
the Postgres binaries are part of neon.tgz again.
neon.tgz artifact in the github workflow included the contents of
'tmp_install', but that seems pointless, because the same files are
included earlier already in the pg.tgz artifact.
