Summary:
Skipping pgrx init call for incremental builds.
cargo pgrx init is required to properly configure the pgrx development environment.
It needs to be only run once, so can be skipped in incremental builds.

This is being done to reduce the yugabyte-db incremental build time.

Test Plan:
Tested locally on macOS, debug build.
Tested incremental build time by simply adding whitespace character in xlogfuncs.c:
./yb_build.sh: ~65 seconds
./yb_build.sh --skip-pg-parquet: ~45 seconds

./yb_build.sh (without the current code changes): ~100 seconds

Reviewers: jason, esheng

Reviewed By: jason

Subscribers: ashetkar, yql

Differential Revision: https://phorge.dev.yugabyte.com/D47760