Since nix 2.20, `nix-store --add-fixed` doesn't accept paths where the
parent directory is a symlink. On macOS, /tmp is a symlink to
/private/tmp, which causes a "'/tmp' is a symlink" error:
```
$ nix run github:nixos/nixpkgs/24.11-beta#nix-prefetch-git -- --url https://github.com/IFTTT/polo.git --rev 316aa2ac210a45a7fc400ab921831493d5dd21b8 --hash sha256
Initialized empty Git repository in /private/tmp/git-checkout-tmp-1Bf9bIv7/polo-316aa2a/.git/
remote: Enumerating objects: 51, done.
remote: Counting objects: 100% (51/51), done.
remote: Compressing objects: 100% (42/42), done.
remote: Total 51 (delta 8), reused 19 (delta 5), pack-reused 0 (from 0)
Unpacking objects: 100% (51/51), 19.57 KiB | 541.00 KiB/s, done.
From https://github.com/IFTTT/polo
* branch HEAD -> FETCH_HEAD
Switched to a new branch 'fetchgit'
removing `.git'...
error: path '/tmp' is a symlink
```
Avoid this by resolving /tmp to a real directory in all the prefetch scripts
Without this change, the `--os` and `--arch` switches are disregarded
for operations involving `skopeo inspect` invocations. This means that,
for example, one cannot fetch Linux images while on macOS.
Informational messages belong on stderr, not on stdout and intermixed
with structured output for programmatic use.
Change-Id: I34d094d04460494e9ec8953db7490f4e2292d959
Jörg Thalheim