Place build_runner.zig in cache directory

[Techatrix]

fixes #634

[SuperAuguste]

LGTM!

[LordMZTE]

Sniped my PR :P

[LordMZTE]

You should append zls to this. It's bad practice to drop stuff straight in ~/.cache.

Also, this is a memory leak since cache_dir_path is never freed.

[llogick]

Also, this is a memory leak since cache_dir_path is never freed.

Yeah, it crops up if you restart a running server

error(gpa): memory address 0x7ff06e67e0d0 leaked: 
zig/lib/std/mem.zig:2182:36: 0x486bdb in std.mem.concatMaybeSentinel (zls)
    const buf = try allocator.alloc(T, total_len);
                                   ^
zig/lib/std/mem.zig:2156:31: 0x37443a in std.mem.concat (zls)
    return concatMaybeSentinel(allocator, T, slices, null);
                              ^
src/known-folders/known-folders.zig:200:38: 0x318943 in .known-folders.getPathXdg (zls)
            return try std.mem.concat(allocator, u8, &[_][]const u8{ home, default[1..] });
                                     ^
src/known-folders/known-folders.zig:129:30: 0x3137f2 in .known-folders.getPath (zls)
            return getPathXdg(allocator, &arena, folder);
                             ^
src/Config.zig:205:54: 0x31e230 in Config.configChanged (zls)
    const cache_dir_path = (try known_folders.getPath(allocator, .cache)) orelse {
                                                     ^
src/Server.zig:2622:36: 0x334f56 in Server.configChanged (zls)
    try server.config.configChanged(server.allocator, builtin_creation_dir);
                                   ^
src/Server.zig:2514:33: 0x32e82e in Server.processJsonRpc (zls)
        try server.configChanged(null);
                                ^
src/main.zig:36:34: 0x30eda1 in loop (zls)
        try server.processJsonRpc(writer, buffer);
                                 ^
src/main.zig:265:13: 0x30a876 in main (zls)
    try loop(&server);
            ^
zig/lib/std/start.zig:578:37: 0x2e08e3 in std.start.posixCallMainAndExit (zls)
            const result = root.main() catch |err| {

[LordMZTE]

Might be best to just move this block above the one creating the build runner file instead of using cache_dir_path.
That would fix the user not being able to change where build_runner.zig is placed.