Fix for `WARNING: SWIFT_CUSTOM_TOOLCHAIN is deprecated. Use --action_… (#3123)

Fixes warning during the usage of custom toolchain

```
Showing All Messages
DEBUG: /private/var/tmp/_bazel_{username}/ed37deb70baf581b96b515b59a8873c2/rules_xcodeproj.noindex/build_output_base/external/rules_swift~/swift/toolchains/xcode_swift_toolchain.bzl:600:14: WARNING: SWIFT_CUSTOM_TOOLCHAIN is deprecated. Use --action_env=TOOLCHAINS=<id> instead.
```

Signed-off-by: Karim Alweheshy <karim.alweheshy@reddit.com>
Co-authored-by: Karim Alweheshy <karim.alweheshy@reddit.com>

Author: karim-alweheshy

xcodeproj/internal/templates/bazel_build.sh

@@ -114,7 +114,7 @@ echo "Starting Bazel build"
   ${build_pre_config_flags:+"${build_pre_config_flags[@]}"} \
   --config="$config" \
   --color=yes \
-  ${toolchain:+--define=SWIFT_CUSTOM_TOOLCHAIN="$toolchain"} \
+  ${toolchain:+--action_env=TOOLCHAINS="$toolchain"} \
   "$output_groups_flag" \
   "%generator_label%" \
   ${labels:+"--build_metadata=PATTERN=${labels[*]}"} \