The runCases function incorrectly called the introspect.findZigLibDir
function, causing a possible error, depending on the location of the
local cache directory, since the current executable is check-case.
Use findZigLibDirFromSelfExe, passing the zig_exe_path argument.
The current CI scripts work correctly because ZIG_LOCAL_CACHE_DIR is set
to "$(pwd)/zig-local-cache".
Fixes#15044