Use tmp context dir when building with process substitution

Podman defaults to the directory of the Containerfile when no context dir is explicitly provided.
When running podman build with process subsituiton,  `podman build -f <(echo "FROM scratch")`,
the Containerfile path expands to `/dev/fd/<NUM>`, which makes `/dev/fd` the context dir.
When building, Buildah attempts to create an overlay mount on top of the `/dev/fd` context dir, which fails.

In these cases, use a temp context dir instead: `$TMPDIR/podman-build-context-$randnum`

Fixes: https://github.com/containers/podman/issues/28113

Signed-off-by: Ashley Cui <acui@redhat.com>