Revert "build: remove some no-longer-needed var unexporting from conf…

Building with depends implies exporting of the PKG_CONFIG_PATH and PKG_CONFIG_LIBDIR environment variables: https://github.com/bitcoin/bitcoin/blob/d1e42659bbdd8da170542d8c638242cd94f71a7d/depends/config.site.in#L87-L93

To use --config-cache option, as we do in CI, requires that "they must be unexported at the end of configure.ac".