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

src/interfaces/libpq/Makefile

@@ -143,11 +143,13 @@ install: all installdirs install-lib
 test-build:
 	$(MAKE) -C test all
 
+check installcheck: export PATH := $(CURDIR)/test:$(PATH)
+
 check: test-build all
-	PATH="$(CURDIR)/test:$$PATH" && $(prove_check)
+	$(prove_check)
 
 installcheck: test-build all
-	PATH="$(CURDIR)/test:$$PATH" && $(prove_installcheck)
+	$(prove_installcheck)
 
 installdirs: installdirs-lib
 	$(MKDIR_P) '$(DESTDIR)$(includedir)' '$(DESTDIR)$(includedir_internal)' '$(DESTDIR)$(datadir)'