build: correctly replace BUILD_SCM_HASH-PLACEHOLDER with BUILD_SCM_ABBREV_HASH

Correctly replace `BUILD_SCM_HASH-PLACEHOLDER` with `BUILD_SCM_ABBREV_HASH` by enclosing it in `{}` for proper interpolation.

Example of the issue: https://github.com/angular/angular-build-builds/blob/cac2e58892c99256d3a1f0abeb274f116edd5a26/package.json#L26

Author: alan-agius4

tools/substitutions.bzl

@@ -5,7 +5,7 @@ _stamp_substitutions = {
     "0.0.0-PLACEHOLDER": "{{STABLE_PROJECT_VERSION}}",
     "0.0.0-EXPERIMENTAL-PLACEHOLDER": "{{STABLE_PROJECT_EXPERIMENTAL_VERSION}}",
     # ---
-    "BUILD_SCM_HASH-PLACEHOLDER": "{BUILD_SCM_ABBREV_HASH}",
+    "BUILD_SCM_HASH-PLACEHOLDER": "{{BUILD_SCM_ABBREV_HASH}}",
     "0.0.0-ENGINES-NODE": RELEASE_ENGINES_NODE,
     "0.0.0-ENGINES-NPM": RELEASE_ENGINES_NPM,
     "0.0.0-ENGINES-YARN": RELEASE_ENGINES_YARN,