sage: fix pkgconfig errors in tests

python.pkgs.pkgconfig raises an exception on missing packages since
version 1.5.0. Previously those errors were just silently ignored. That
worked fine, since the packages are only missing at runtime (when they
are not really needed) but present at buildtime.

Since this fails the tests now, we just add the packages to
PKG_CONFIG_PATH at runtime. This does not add additional runtime
dependencies. Still, it would be nicer if the sage testssuite would not
test the buildsystem at runtime in the first place.

The breakage was originally caused by the pkgconfig update in
1efa7161.