Handle TEMP_CONFIG for pg_regress style tests in pg_regress.c

Hi,

When building with meson, TEMP_CONFIG is supported for TAP tests, but doesn't
do anything for regress/isolation.

The reason for that is that meson's (and ninja's) architecture is to separate
"build setup" from the "build/test/whatever" stage, moving dynamism (and more
costly operations) to the "setup" phase.

In this case the implication is that the command line for the test isn't
re-computed dynamically. But pg_regress doesn't look at TEMP_CONFIG, it just
has a --temp-config=... parameter, that src/Makefile.global.in dynamically
adds if TEMP_CONFIG is set.

In contrast to that, TEMP_CONFIG support for tap tests is implemented in
Cluster.pm, and thus works transparently.

My inclination is to move TEMP_CONFIG support from the Makefile to
pg_regress.c. That way it's consistent across the build tools and isn't
duplicated. pg_regress already looks at a bunch of temporary variables
(e.g. PG_REGRESS_SOCK_DIR, PG_TEST_USE_UNIX_SOCKETS), so this isn't really
breaking new ground.

It can be implemented differently, e.g. by adding the parameter dynamically in
the wrapper around pg_regress, but I don't see an advantage in that.

Patch attached.

Greetings,

Andres Freund