Stop rustfmt versioning from overriding rust-analyzer versioning.

[ReticentIris]

There is a bug/typo in rust_register_toolchains where rustfmt_version would always override rust_analyzer_version (default or otherwise specified). This causes problems related to mismatched ABIs when rust-analyzer tries to expand proc macros if one were to use an older-than-default nightly toolchain without specifying a specific rustfmt_version. This is because rust-analyzer will be set to use the default rustfmt_version which is nightly/2024-02-08:

rules_rust/rust/private/common.bzl

Line 36 in 8b977b5

DEFAULT_NIGHTLY_ISO_DATE = "2024-02-08"

As a very basic example of what I mean:

rust_register_toolchains(
    edition = "2021",
    versions = [
        "nightly/2024-01-31",
    ],
    extra_exec_rustc_flags = ["-A", "async_fn_in_trait"],
)

Will result in rust-analyzer complaining with:

proc macro `Deserialize` not expanded: Cannot create expander for /private/var/tmp/_bazel_a/a03e1cfb3989a3cda645602bbe0ee892/execroot/_main/bazel-out/darwin_arm64-opt-exec-ST-13d3ddad9198/bin/external/crate_index__serde_derive-1.0.195/libserde_derive-1365269244.dylib: mismatched ABI expected: `rustc 1.78.0-nightly (8ace7ea1f 2024-02-07)`, got `rustc 1.77.0-nightly (11f32b73e 2024-01-31)`rust-analyzer[unresolved-proc-macro](https://rust-analyzer.github.io/manual.html#unresolved-proc-macro)

Using a workaround of setting rustfmt_version causes the ABI mismatch to be resolved:

rust_register_toolchains(
    edition = "2021",
    versions = [
        "nightly/2024-01-31",
    ],
    rustfmt_version = "nightly/2024-01-31",
    extra_exec_rustc_flags = ["-A", "async_fn_in_trait"],
)

This PR fixes the issue so that rust-analyzer version selection actually works.

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[UebelAndre]

Good catch, thank you!