fix(ng-dev): always emit project version info as stable stamping keys

This fixes a long-standing issue where the project version is
emitted as a volatile key in stamping. This means that everytime a
new version is put into `package.json`, the NPM package will not
be rebuilt if it has been built previously.

This is the typical source of outdated NPM package artifacts where
the package version is accidentally still the old one. The Bazel
idiomatic fix is to use a `STABLE_` stamping key so that Bazel will
properly re-build dependent targets whenever the version changes.

Ideally as part of larger future Bazel refactorings we would also
make sure that only necessary target configurations depend on the
stable status key file. Currently `ng_package`/`pkg_npm` always
depends on it, even if there is no subsitution dependency on actual
stable keys.

bazel-contrib/rules_nodejs#1219 (comment)

Angular currently always wipes the cached artifact exactly to workaround
this issue. Previously it even built completely from scatch, dismissing
all of the previously cached artifacts.

ng-dev/release/stamping/env-stamp.ts

@@ -29,8 +29,12 @@ export async function printEnvStamp(mode: EnvStampMode, includeVersion: boolean)
 
   if (includeVersion === true) {
     const {version, experimentalVersion} = getSCMVersions(git, mode);
-    console.info(`BUILD_SCM_VERSION ${version}`);
-    console.info(`BUILD_SCM_EXPERIMENTAL_VERSION ${experimentalVersion}`);
+    // Note: We need to use the `STABLE_` prefix to tell Bazel that these
+    // variables are changing rarely, and if they do- the targets relying
+    // on it should be rebuilt. e.g. the NPM package would need to be re-assembled.
+    // https://bazel.build/docs/user-manual#workspace-status-command.
+    console.info(`STABLE_PROJECT_VERSION ${version}`);
+    console.info(`STABLE_PROJECT_EXPERIMENTAL_VERSION ${experimentalVersion}`);
   }
 }