fix(std.Build.Step.Run): properly cache input directories.

[GasInfinity]

It applies the same logic std.Build.Step.WriteFile uses. However some run steps want the cache directory as an argument thus breaking things.

Closes #20935

[GasInfinity]

😅 Oops, I did a simple test locally and it worked. I'll check CI failures later, sorry!

[GasInfinity]

Looks like this was what triggered the failures, trying to cache the cache directory...

zig/lib/std/Build/Step/Run.zig

Lines 229 to 237 in e0dc2e4

	pub fn enableTestRunnerMode(run: *Run) void {
	const b = run.step.owner;
	run.stdio = .zig_test;
	run.addPrefixedDirectoryArg("--cache-dir=", .{ .cwd_relative = b.cache_root.path orelse "." });
	run.addArgs(&.{
	b.fmt("--seed=0x{x}", .{b.graph.random_seed}),
	"--listen=-",
	});
	}

The tmp directory gets added to the cache and then it gets deleted, thus causing those FileNotFound errors.

One simple fix is ignoring non-existent files when populating hashes. However I don't think caching the cache directory is a good idea.