zig launcher: replace shell wrappers with a binary

Until now we needed to maintain two versions of the zig launcher: one
for Windows and one for everything else. This was problematic for two
reasons:
1. I do not know powershell and thus keep breaking the Windows wrapper
   all the time (see git history of Fabian fixing stuff that I broke).
2. This makes bazel-zig-cc dependent on the system shell, making it not
   really hermetic. So the recently added
   `--experimental_use_hermetic_linux_sandbox` does not work with
   bazel-zig-cc, unless we bind-mount a bunch of stuff: `/usr`, `/bin`,
   `/lib`, `/usr/lib:/lib`, `/usr/lib64:/lib64` and `/proc`.

Switching to a Zig-based wrapper solves both issues, and we can do this:

    bazel build "$@" \
        --experimental_use_hermetic_linux_sandbox \
        --sandbox_add_mount_pair=/proc \
        <...>

Zig itself still depends on `/proc` for `/proc/self/exe`, so we need to
keep that. I will look into reducing even that dependency separately.

Not all is nice and shiny though: this commit replaces ~80 LOC worth of
shell scripts wrappers with a singe ~300 LOC zig program, which is
arguably harder to understand. However, it is easier to change, at least
for me, because it's a single file with unit tests! Most importantly,
the gnarly code (which resolves paths and sets environment variables) is
cross-platform.

Thanks to Fabian Hahn for testing this on Windows and pointing out
errors.

ci/launcher

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -xeuo pipefail
+
+ZIG=${ZIG:-bazel run "$@" @zig_sdk//:zig --}
+
+# until bazel-zig-cc gets a zig toolchain, run launcher's unit tests here.
+$ZIG test toolchain/launcher.zig
+
+# ReleaseSafe because of https://github.com/ziglang/zig/issues/14036
+$ZIG test \
+    -OReleaseSafe \
+    -target x86_64-windows-gnu \
+    --test-cmd wine64-stable \
+    --test-cmd-bin \
+    toolchain/launcher.zig