Buildifier reformats assignment ignoring "# buildifier: leave-alone"

Hi, I have a `.bzl` file that I need to keep compatible with both Starlark and Bash

The original contents of the file:

```
# buildifier: leave-alone
MY_VAR="value"
```

However after running `buildifier -v -lint fix -mode fix` against this file (we do that as pre-commit hook) it still reformats the assignment by adding _spaces_ aroud `=`:

```
# buildifier: leave-alone
MY_VAR = "value"
```

` = ` is invalid in Bash :(

As mentioned here https://github.com/bazelbuild/buildtools/issues/806#issuecomment-600644003 https://github.com/bazelbuild/buildtools/blob/195b83e94906df245fdc44da27b1be5489087025/build/rewrite.go#L128-L129, I used `buildifier: leave-alone` comment, but it still reformats it.

Looks like a bug, wdyt?

---

Notes:

- I've tried to find a category that reformatting falls in, but couldn't figure out after reading https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#buildifier-warnings
- I've tried to make `buildifier` print JSON to figure out the category, but that doesn't seem to work either:
```console
buildifier --format=json --type=check myfile.bzl
buildifier: unrecognized input type check; valid types are build, bzl, workspace, default, auto
```
I'll submit a PR for this, should be `--mode=check` it seems (submitted PR https://github.com/bazelbuild/buildtools/pull/891)

Running on macOS

```
buildifier version: 3.4.0
buildifier scm revision: b1667ff58f714d13c2bba6823d6c52214705508f
```

Start in build/rewrite.go around the referenced lines 128-129 and reproduce the behavior with the shown .bzl input and buildifier command. Verify that the `# buildifier: leave-alone` directive prevents assignment reformatting, then run the relevant formatter checks to confirm the corrected behavior.

Written by the indexing model from the issue text.