python3: fix host PGO build failure on deep build paths

python3: fix host PGO build failure on deep build paths

Python 3.14 made "forkserver" the default multiprocessing start method on
Linux, which binds an AF_UNIX socket under $TMPDIR during the PGO profile-run.
OpenWrt points TMPDIR at the deeply nested build tree, so the socket path can
exceed the 108-byte AF_UNIX limit and abort the host build with "AF_UNIX path
too long" in test_re (cpython#149527). Pin TMPDIR=/tmp for the host build.

Signed-off-by: Alexandru Ardelean <redacted>