CLI: improved local cache directory logic

Previously, when choosing the local cache directory, if there was no
root source file, an explicitly chosen path, or other clues, zig would
choose cwd + zig-cache/ as the local cache directory.

This can be problematic if Zig is invoked with the CWD set to a
read-only directory, or a directory unrelated to the actual source files
being compiled. In the real world, we see this when using `zig cc` with
CGo, which for some reason changes the current working directory to the
read-only go standard library path before running the C compiler.

This commit conservatively chooses to use the global cache directory
as the local cache directory when there is no other reasonable choice,
and no longer will rely on the cwd path to choose a local cache directory.

As a reminder, the --cache-dir CLI flag and ZIG_LOCAL_CACHE_DIR
environment variable are available for overriding the decision. For the
zig build system, it will always choose the directory that build.zig is
+ zig-cache/.

Closes #7342

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