environmentalDependencyCommands

Sometimes metadata about the environment is an important input to the build process (for instance, to include a `Built at c9e6ee2` comment at the top of a bundled file).

Presently, wireit will return a cached output if the input files haven't changed, even if the environment has. There's no way to declare an explicit dependency on the environment. I propose we change that, with something like this:

```json
{
"wireit": {
"build": {
"command": "vite build",
"files": [
"dist/**/index.html",
"dist/**/*.js",
"vite.config.ts"
],
"environmentalDependencyCommands": [
"git rev-parse HEAD"
]
}
}
}
```

where the results of each entry in `environmentalDependencyCommands` would be cached, and a rebuild would be triggered if any of them changed (just like any other `dependency`).

In this case, making a git commit would change `HEAD`, which would invalidate the build even if the enumerated files haven't changed.