New file name for zig cache directory '.zig-cache'

the gitignore for zig doesn't filter out the new name for the cache directory, gitignore currently it generates:

```sh
### zig ###
# Zig programming language

zig-cache/ # <---
zig-out/
build/
build-*/
docgen_tmp/
```
while zig outputs:

```sh
.
├── build.zig
├── build.zig.zon
├── src
├── .zig-cache # <---
└── zig-out
```