feat: support toolchains building zig from source

[cerisier]

As explained in #644

This PR makes it possible to build zig from source as part of the toolchain.
The way it works is by allowing end users to define their own:

zig_toolchain(
    name = "zig_from_source",
    zig_cache = "/var/tmp/zig-cache",
    zig_exe = "@zig//:zig",
    zig_h = "@zig//:zig_h",
    zig_lib = "@zig//:lib_dir",
    zig_version = "0.16.0",
)

toolchain(
    name = "zig-source-toolchain"
    target_settings = [
        "@rules_zig//zig/config/bootstrapped:bootstrap_enabled",
    ],
    toolchain = ":zig_from_source",
    toolchain_type = "@rules_zig//zig:toolchain_type",
    target_compatible_with = [...],
)

and passing a new build flat --@rules_zig//zig/settings:bootstrapped=True.

This setting is used in a rule transition for zig_toolchain, setting bootstrapped=False so that all dependencies of zig_toolchain if they are zig targets, do not resolve the same zig_from_source toolchain but the prebuilt toolchain which doesn't have the bootstrapped target_settings.

---

However, this PR currently breaks compatibility with path-based non hermetic zig installations.

Supporting local installation is very easy with this setup if we use new_local_repository with symlinks but I remember your objections wrt windows.

I wonder if there is a way to somehow split the logic entirely so that we don't end up with 1 toolchain API bent to support 2 conflicting use case.

Another argument to keep passing File around in the nominal case is to support path mapping which prohibits passing .path to actions.