zig run should perform an execve on operating systems that support it

[andrewrk]

This is a minor regression introduced from #6250. zig run and zig test run the binary as a child process. Instead, on operating systems that support it, they should perform an execve, which is more resource-efficient and has nicer properties when using zig as a child process for these use cases.

zig/src/main.zig

Lines 1684 to 1687 in 52ba2c3

	// TODO On operating systems that support it, do an execve here rather than child process,
	// when watch=false and arg_mode == .run
	const child = try std.ChildProcess.init(argv.items, gpa);
	defer child.deinit();

[andrewrk]

Note that zig test should not because we want to print the error message and command line on failure.