build: add Bazel Rust workflow

bazel/utils.bzl

@@ -0,0 +1,12 @@
+def require_env(repository_ctx, variable):
+    value = repository_ctx.os.environ.get(variable)
+    if not value:
+        fail("{} must be set.".format(variable))
+    return value
+
+
+def execute(repository_ctx, arguments):
+    result = repository_ctx.execute(arguments)
+    if result.return_code != 0:
+        fail("command failed: {}\n{}".format(" ".join(arguments), result.stderr))
+    return result.stdout.strip()