ci: use valid version number during CI build

Just a "ci-fixed" will be invalid in the debian packages.

.buildkite/hooks/pre-command

@@ -30,9 +30,10 @@ rm -f $HOME/.bazelrc
 # --nostamp is required for better caching (only on non-release jobs).
 if [ "$BUILDKITE_PIPELINE_SLUG" == "scion" ]; then
     echo "build --nostamp" > $HOME/.bazelrc
-    # Also set a fixed GIT_VERSION so that the workspace_status_command always
-    # returns the same value on CI to improve cache reuse.
-    export GIT_VERSION="ci-fixed"
+    # Shorten the git version to omit commit information, improving cache reuse.
+    # The format of git-version is "<tag>-<number-of-commits-since-the-tag>-<commit-short-hash>"
+    # This will be shortened to "<tag>-modified-ci"
+    export GIT_VERSION=$(tools/git-version | sed 's/-.*/-modified-ci/')
 else
     echo "build --stamp" > $HOME/.bazelrc
 fi