zig cache dir: move to /tmp/zig-cache (#52)

This cache directory can be re-used by everything and everyone. There is
nothing bazel or hermetic_cc_toolchain specific there. So let's make it
clear.

Also, if there are any more zig-based toolchains on top of Bazel or
other build systems where they cannot rely on $HOME, but need an
absolute path, this feels like a reasonable choice.

ci/test

@@ -5,7 +5,7 @@
 
 set -xeuo pipefail
 
-cache_prefix="${HERMETIC_CC_TOOLCHAIN_CACHE_PREFIX:-/tmp/hermetic_cc_toolchain}"
+cache_prefix="${HERMETIC_CC_TOOLCHAIN_CACHE_PREFIX:-/tmp/zig-cache}"
 
 # check a very hermetic setup with a single target. Re-building all of
 # them takes a long time, so using only one. If we ever decide to build all

toolchain/defs.bzl

@@ -154,7 +154,7 @@ def _zig_repository_impl(repository_ctx):
         if os == "windows":
             cache_prefix = "C:\\\\Temp\\\\hermetic_cc_toolchain"
         else:
-            cache_prefix = "/tmp/hermetic_cc_toolchain"
+            cache_prefix = "/tmp/zig-cache"
 
     repository_ctx.template(
         "tools/launcher.zig",