Fix [install]check in interfaces/libpq/Makefile
The common recipe when TAP tests are disabled doesn't work, because the
libpq-specific recipe wants to define the PATH environment variable, so
the starting '@' is misinterpreted as part of the command instead of
silencing said command.
Fix by setting the environment variable in a way that doesn't interfere
with the recipe.
Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/
20220720172321.GL12702@telsasoft.com