gallium/pipe: Add a GALLIUM_PIPE_SEARCH_DIR override env var

This can be useful if you rsync an install between two machines and the
paths don't perfectly match up.  OpenGL drivers already work fine but
anything which uses pipe-loader has a compile-time path.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7047>
3 files changed