devel/electron37: unbreak non-root build with poudriere

The chromium source archive hosted at GitHub is splitted into multiple
fragments due to size constraints. We have to concatenate them into a
single tarball before extracting it.

Previously, the combined tarball was written to DISTDIR/DIST_SUBDIR,
which may not be writable by the poudriere user. So change the
directory to WRKSRC:H and manually extracting the tarball to avoid
such breakage.

While here, replace WRKDIR/PORTNAME-ELECTRON_VER with WRKSRC:H for
better consiceness and readability.

PR: 289226
Reported by: Dan Kotowski <dan.kotowski@a9development.com>
Tested by: Dan Kotowski <dan.kotowski@a9development.com>