Auto merge of #15996 - servo:incremental-unstability, r=Manishearth

Fix "option `Z` is unstable" warning on geckolib with incremental=true

`incremental = true` can be set in the `[build]` section of `.servobuild` to enable incremental compilation.

In `./mach build-geckolib`, which uses a stable version of the compiler, this would cause a warning repeated for each invocation of rustc:

```
warning: the option `Z` is unstable and should only be used on the nightly
compiler, but it is currently accepted for backwards compatibility;
this will soon change, see issue #31847 for more details
```

This PR disables incremental compilation for `build-geckolib`, regardless of `.servobuild` settings.

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