Use Rust 1.79 for Bazel and nightly for Cargo

We need to use a recent nightly release to be able to use the `--workspace`
flag to `cargo publish`. Our minimum supported Rust version is 1.79,
so I think we should test that version with Bazel to make sure we stay
compatible. Our Bazel build is currently using 1.86 since that is the default
from rules_rust, but this change explicitly selects 1.79.

I also tried having Bazel use a more recent version, but unfortunately
rules_rust is broken for 1.87 and up, due to name mangling changes in rustc:
bazelbuild/rules_rust#3459

As soon as Rust 1.90 is released, we can start using that with Cargo instead of
nightly.

PiperOrigin-RevId: 793660982

Author: acozzette

.github/workflows/test_rust.yml

@@ -30,7 +30,10 @@ jobs:
 
           # Override cases with custom images
           - config: { name: Cargo }
-            image: "us-docker.pkg.dev/protobuf-build/containers/release/linux/rust:8.0.1-1.79.0-d271543f317955b5732ce19b2be672a195e96508"
+            # We currently need to be on nightly to have access to the
+            # multi-package publishing feature. This will be stabilized in
+            # 1.90, so we should switch to that as soon as it is released.
+            image: "us-docker.pkg.dev/protobuf-build/containers/release/linux/rust:8.0.1-nightly-2025-08-07-f0d1e209ed9369f69d93ce418990ecff3aa08d6f"
             bazel_cmd: "run"
             targets: "//rust/release_crates:cargo_test"
 

MODULE.bazel

@@ -180,6 +180,12 @@ register_toolchains(
     dev_dependency = True,
 )
 
+rust = use_extension("@rules_rust//rust:extensions.bzl", "rust", dev_dependency = True)
+rust.toolchain(
+    edition = "2021",
+    versions = [ "1.79.0" ],
+)
+
 crate = use_extension("@rules_rust//crate_universe:extension.bzl", "crate", dev_dependency = True)
 crate.spec(
     package = "googletest",