Initialize Cargo / timescaledb only if available

Previously, we depended on the value of PG_VERSIONS or TSDB_PG_VERSIONS.
This doesn't always work correctly, however, we can switch to
determining the versions at runtime, that should prevent errors like:

     528.1 + for pg in '$PG_VERSIONS'
     528.1 + init_flags=' --pg12 /usr/pgsql-12/bin/pg_config --pg13 /usr/pgsql-13/bin/pg_config --pg14 /usr/pgsql-14/bin/pg_config --pg15 /usr/pgsql-15/bin/pg_config --pg16 /usr/pgsql-16/bin/pg_config'
     528.1 + cargo pgrx init --pg12 /usr/pgsql-12/bin/pg_config --pg13 /usr/pgsql-13/bin/pg_config --pg14 /usr/pgsql-14/bin/pg_config --pg15 /usr/pgsql-15/bin/pg_config --pg16 /usr/pgsql-16/bin/pg_config
     528.1      Creating PGRX_HOME at `/home/postgres/.pgrx`
     528.1 The application panicked (crashed).
     528.1 Message:  no major version:
     528.1    0: The specified pg_config binary, `/usr/pgsql-16/bin/pg_config`, does not exist
     528.1    1: No such file or directory (os error 2)

docker/ci/setup.sh

@@ -252,13 +252,19 @@ cargo install cargo-pgrx --version =$PGRX_VERSION
 
 # Configure pgrx
 ## `cargo pgrx init` is not additive; must specify all versions in one command.
-for pg in $PG_VERSIONS; do
-    init_flags="$init_flags --pg$pg $PG_BASE$pg/bin/pg_config"
-done
+for pg_config in $(find /usr -name 'pg_config' -type f | grep "${PG_BASE}"); do
+    pg="$(${pg_config} --version | awk -F '[ .]' '{print $2'})"
+    init_flags="$init_flags --pg$pg ${pg_config}"
 cargo pgrx init $init_flags
-## Initialize pgrx-managed databases so we can add the timescaledb load.
-for pg in $TSDB_PG_VERSIONS; do
-    echo "shared_preload_libraries = 'timescaledb'" >> ~/.pgrx/data-$pg/postgresql.conf
+
+## Initialize pgrx-managed databases so we can add the timescaledb load, but only
+## for those PostgreSQL versions that have the timescaledb.so library available.
+for pg_config in $(find /usr -name 'pg_config' -type f | grep "${PG_BASE}"); do
+    pg="$(${pg_config} --version | awk -F '[ .]' '{print $2'})"
+    lib="$(find "${PGBASE}${pg}" -type f -name 'timescaledb.so')"
+    if [ "${lib}" != "" ]; then
+        echo "shared_preload_libraries = 'timescaledb'" >> ~/.pgrx/data-$pg/postgresql.conf
+    fi
 done
 
 # Clone and fetch dependencies so we builds have less work to do.