Disable rmake test rustdoc-io-error on riscv64gc-gnu
In https://github.com/rust-lang/rust/pull/126917 we disabled `inaccessible-temp-dir` on `riscv64gc-gnu` because the container runs the build as `root` (just like the `armhf-gnu` builds). Tests creating an inaccessible test directory are not possible, since `root` can always touch those directories.
553a69030e/src/ci/docker/host-x86_64/disabled/riscv64gc-gnu/Dockerfile (L99)
This means the tests are run as `root`. As `root`, it's perfectly normal and reasonable to violate permission checks this way:
```bash
$ sudo mkdir scratch
$ sudo chmod o-w scratch
$ sudo mkdir scratch/backs
$
```
Because of this, this PR makes the test ignored on `riscv64gc` (just like on `armhf-gnu`) for now.
As an alternative, I believe the best long-term strategy would be to not run the tests as `root` for this job. Some preliminary exploration was done in https://github.com/rust-lang/rust/pull/126917#issuecomment-2189933970, however that appears a larger lift.
## Testing
> [!NOTE]
> `riscv64gc-unknown-linux-gnu` is a [**Tier 2 with Host Tools** platform](https://doc.rust-lang.org/beta/rustc/platform-support.html), all tests may not necessarily pass! This change should only ignore `inaccessible-temp-dir` and not affect other tests.
You can test out the job locally:
```sh
DEPLOY=1 ./src/ci/docker/run.sh riscv64gc-gnu
```
r? `@jieyouxu`
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