PSA: fx run-boot-test --build

[Roland McGrath]

As of fxrev.dev/1093312 the `fx run-boot-test` command now accepts a `--build` switch.  This will make it rebuild precisely what's needed for the selected boot test and not (much) more.  Without the switch, it will do that build if the image doesn't exist at all (this behavior is not new), but `--build` tells it to make sure it's up to date with an incremental `fx build <test-label>`.

Note that until somewhat recently, `fx run-boot-test` did a build by default because `fx qemu` did a build (though not necessarily exactly the build you needed).  `fx qemu` no longer supports `--build` and there are no current plans to change that.  Before that support was removed, `fx run-boot-test --args=--no-build` was the way to do `fx run-boot-test` without doing any incremental rebuilding implicitly.

If consensus prefers it, we may change `fx run-boot-test` to do `--build` by default in the (perhaps near) future.

Now and in the future `fx run-boot-test --no-build` can be used for a workflow without implicit building, and `--build` can be used to ensure implicit building.

Enjoy your boot test running!