CI: Fix path confusion in OpenCL Piglit execution

CI: Fix path confusion in OpenCL Piglit execution

When we remove the contents of the results directory, we `cd` into it.
The script expects that $PWD is /piglit, and $OLDPWD is the Mesa build
directory, however the cd into the results directory will make $OLDPWD
be $BUILDDIR/results.

This means that Piglit emits into results/results/ which looks weird,
but more importantly also fails OpenCL Piglit execution, because we
can't find our baseline result expectations.

Fix it by using an explicit variable rather than relying on history.

Fixes: 683ddf19dc85 ("ci: remove results directory content only with piglit runners")
Ref: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10856
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Martin Peres <martin.peres@mupuf.org>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11126>