Release: Replace legacy backticks with $() notation

Fix this shellcheck SC2006 warning accordingly.

Change-Id: I2babca84f12bd3fd27fe687490ac84527d1cb54b
diff --git a/jenkins-docker/slave-release/gerrit-release.sh b/jenkins-docker/slave-release/gerrit-release.sh
index 7526a9d..ea0a01b 100755
--- a/jenkins-docker/slave-release/gerrit-release.sh
+++ b/jenkins-docker/slave-release/gerrit-release.sh
@@ -24,7 +24,7 @@
 fi
 
 echo "Cloning and building Gerrit Code Review on branch $branch ..."
-git clone https://gerrit.googlesource.com/gerrit && (cd gerrit && f=`git rev-parse --git-dir`/hooks/commit-msg ; curl -Lo $f https://gerrit-review.googlesource.com/tools/hooks/commit-msg ; chmod +x $f)
+git clone https://gerrit.googlesource.com/gerrit && (cd gerrit && f=$(git rev-parse --git-dir)/hooks/commit-msg ; curl -Lo $f https://gerrit-review.googlesource.com/tools/hooks/commit-msg ; chmod +x $f)
 
 pushd gerrit