Prepend PKG_CONFIG_PATH in when_build_dependency.

This helps avoiding accidents where we accidentally use
a system package despite specifying a local package.

It seems to me that we always want to prefer the pkg-config of hashdist packages over system packages. Would it make sense to add a `base/pkgconfig.yaml` for all packages that use pkg-config? Maybe even auto-detect this in autotools_package? Or just generally export PKG_CONFIG_PATH?