ci/angle: Use relative paths

When we use build-angle as a sourced script, $0 refers to the overall
build script, which may not be in .gitlab-ci/container/.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31781>

.gitlab-ci/container/build-angle.sh

@@ -7,7 +7,7 @@
 set -uex
 
 ANGLE_REV="76025caa1a059f464a2b0e8f879dbd4746f092b9"
-SCRIPTS_DIR="$(realpath "$(dirname "$0")/..")"
+SCRIPTS_DIR="$(pwd)/.gitlab-ci"
 ANGLE_PATCH_DIR="${SCRIPTS_DIR}/container/patches"
 
 # DEPOT tools