Add support to build hermes from a specific commit (#36681)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36681

This change is needed to add some E2E tests on the Hermes repo so that we can test Hermes against iOS other than android.

## Changelog:
[Internal][Added] - Add possibility to download Hermes from a specific commit

Reviewed By: cortinico

Differential Revision: D44460479

fbshipit-source-id: 89c196aa7c38533e4904444f7f17eb3236fc6356

packages/react-native/sdks/hermes-engine/hermes-utils.rb

@@ -35,6 +35,8 @@ def compute_hermes_source(build_from_source, hermestag_file, git, version, build
 
     if ENV.has_key?('HERMES_ENGINE_TARBALL_PATH')
         use_tarball(source)
+    elsif ENV.has_key?('HERMES_COMMIT')
+        build_hermes_from_commit(source, git, ENV['HERMES_COMMIT'])
     elsif build_from_source
         if File.exist?(hermestag_file)
             build_from_tagfile(source, git, hermestag_file)
@@ -131,6 +133,12 @@ def build_hermes_from_source(source, git)
     source[:commit] = `git ls-remote https://github.com/facebook/hermes main | cut -f 1`.strip
 end
 
+def build_hermes_from_commit(source, git, commit)
+    putsIfPodPresent("[Hermes] Installing hermes-engine from commit #{commit}. It may take a while.")
+    source[:git] = git
+    source[:commit] = commit
+end
+
 # This function checks that Hermes artifact exists.
 # As of now it should check it on the Maven repo.
 #