Fix [install]check in interfaces/libpq/Makefile

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 <[email protected]>
Discussion: https://postgr.es/m/20220720172321[email protected]