launcher miscompilation workaround

Sometimes the launcher fails to compile with the following error
messsage:

```
error: FileNotFound
```

We cannot reproduce this in a controlled environment, but see it
happening in the wild often enough to receive repeated questions.

Since this has been escalated to Zig Software Foundation, the most
meaningful thing we can ask our users to do is apply a workaround and
wait. Let's do just that.

toolchain/defs.bzl

@@ -42,6 +42,25 @@ _HOST_PLATFORM_EXT = {
     "windows-x86_64": "zip",
 }
 
+_compile_failed = """
+Compilation of launcher.zig failed:
+command={compile_cmd}
+return_code={return_code}
+stderr={stderr}
+stdout={stdout}
+
+You most likely hit a rare but known race in Zig SDK. Congratulations?
+
+We are working on fixing it with Zig Software Foundation. If you are curious,
+feel free to follow along in https://github.com/ziglang/zig/issues/14815
+
+There isn't much to do now but wait. Now apply the following workaround:
+$ rm -fr {cache_prefix}
+$ <... re-run your command ...>
+
+... and proceed with your life.
+"""
+
 def toolchains(
         version = _VERSION,
         url_formats = [URL_FORMAT_BAZELMIRROR, URL_FORMAT_NIGHTLY],
@@ -173,24 +192,30 @@ def _zig_repository_impl(repository_ctx):
         },
     )
 
+    compile_env = {
+        "ZIG_LOCAL_CACHE_DIR": cache_prefix,
+        "ZIG_GLOBAL_CACHE_DIR": cache_prefix,
+    }
+    compile_cmd = [
+        paths.join("..", "zig"),
+        "build-exe",
+        "-OReleaseSafe",
+        "launcher.zig",
+    ] + (["-static"] if os == "linux" else [])
+
     ret = repository_ctx.execute(
-        [
-            paths.join("..", "zig"),
-            "build-exe",
-            "-OReleaseSafe",
-            "launcher.zig",
-        ] + (["-static"] if os == "linux" else []),
+        compile_cmd,
         working_directory = "tools",
-        environment = {
-            "ZIG_LOCAL_CACHE_DIR": cache_prefix,
-            "ZIG_GLOBAL_CACHE_DIR": cache_prefix,
-        },
+        environment = compile_env,
     )
     if ret.return_code != 0:
-        fail("compilation failed:\nreturn_code={}\nstderr={}\nstdout={}".format(
-            ret.return_code,
-            ret.stdout,
-            ret.stderr,
+        full_cmd = [k + "=" + v for k, v in compile_env.items()] + compile_cmd
+        fail(_compile_failed.format(
+            compile_cmd = " ".join(full_cmd),
+            return_code = ret.return_code,
+            stdout = ret.stdout,
+            stderr = ret.stderr,
+            cache_prefix = cache_prefix,
         ))
 
     exe = ".exe" if os == "windows" else ""