chore(ci): conditionally stamp genrules (#63204)

See https://github.com/bazelbuild/bazel/releases/tag/7.1.0 which
introduces a new value for the stamp attribute for `genrule`: `stamp =
-1` which makes the stamping conditional.

Previously, any genrule that had `stamp = 1` would be stamping all the
time due to how it was implemented prior 7.1. An as we stamp builds with
a computed version that is date dependent, we would never get cache hits
on those, leading to `//cmd/migrator:image_test` being executed all the
time. It wasn't that big of an issue as it's rather quick, but still.

Now the `genrule` supports that there's no reason to fix that one. 


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CI 

## Changelog

- Avoid invalidating cache in stamped genrules on dev builds.

cmd/migrator/BUILD.bazel

@@ -63,7 +63,7 @@ genrule(
         tar -cf $@ --owner=:0 --group=:0 --numeric-owner schema-descriptions/
     fi
     """,
-    stamp = 1,
+    stamp = -1,
     visibility = ["//visibility:public"],
 )
 

cmd/migrator/airgappedgen/BUILD.bazel

@@ -71,7 +71,7 @@ genrule(
         tar -cf $@ --owner=:0 --group=:0 --numeric-owner schema-descriptions/
     fi
     """,
-    stamp = 1,
+    stamp = -1,
     tags = ["requires-network"],
     tools = [":airgappedgen"],
     visibility = ["//cmd/migrator:__subpackages__"],