ci/nouveau: Disable the gm20b jobs entirely.

The old (broken, hw failure) board isn't really hooked up right now, and
I'm waiting on buying some remaining bits to set up the 10 boards sitting
in the box here.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21205>

src/gallium/drivers/nouveau/ci/gitlab-ci.yml

@@ -50,7 +50,7 @@ gk20a-gles:
     DEQP_SUITE: nouveau-gk20a
 
 # Single Jetson Nano board at anholt's house.
-gm20b-gles-full:
+.gm20b-gles-full:
   extends:
     - .gm20b-test
     - .nouveau-manual-rules
@@ -60,9 +60,9 @@ gm20b-gles-full:
     DEQP_SUITE: nouveau-gm20b
     TEST_PHASE_TIMEOUT: 120
 
-gm20b-gles:
+.gm20b-gles:
   extends:
-    - gm20b-gles-full
+    - .gm20b-gles-full
   timeout: 60m
   variables:
     DEQP_FRACTION: 10