Revert "Travis: Disable the Rust macOS build"

This reverts commit 4c4a0d50.

.travis.yml

@@ -63,10 +63,9 @@ matrix:
     ## We check disable module dirauth
     - env: MODULES_OPTIONS="--disable-module-dirauth"
     ## We run rust on macOS, because we have seen macOS rust failures before
-    # Disabled due to slow Travis macOS builds, see #32177
-    #- env: RUST_OPTIONS="--enable-rust --enable-cargo-online-mode"
-    #  compiler: clang
-    #  os: osx
+    - env: RUST_OPTIONS="--enable-rust --enable-cargo-online-mode"
+      compiler: clang
+      os: osx
     ## We run chutney on macOS, because macOS Travis has IPv6
     # Disabled due to slow Travis macOS builds, see #32177
     #- env: CHUTNEY="yes" CHUTNEY_ALLOW_FAILURES="2" SKIP_MAKE_CHECK="yes"