use-pt-pinned-commit for test-arm-{backend,reference}-delegation

Without this, these builds don't respect the torchgen pinned commit and thus fail with #7546.

Differential Revision: [D67996459](https://our.internmc.facebook.com/intern/diff/D67996459/)

[ghstack-poisoned]

Author: swolchok

.github/workflows/trunk.yml

@@ -144,7 +144,7 @@ jobs:
         conda activate "${CONDA_ENV}"
 
         source .ci/scripts/utils.sh
-        install_executorch
+        install_executorch "use-pt-pinned-commit"
 
         .ci/scripts/setup-arm-baremetal-tools.sh
 
@@ -171,7 +171,7 @@ jobs:
         conda activate "${CONDA_ENV}"
 
         source .ci/scripts/utils.sh
-        install_executorch
+        install_executorch "use-pt-pinned-commit"
 
         .ci/scripts/setup-arm-baremetal-tools.sh