build-support: fix nix-prefetch-* on macOS

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

pkgs/build-support/docker/nix-prefetch-docker

@@ -121,7 +121,7 @@ fi
 
 sourceUrl="docker://$imageName@$imageDigest"
 
-tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/skopeo-copy-tmp-XXXXXXXX")"
+tmpPath="$(mktemp -d "$(realpath "${TMPDIR:-/tmp}/skopeo-copy-tmp-XXXXXXXX")")"
 trap "rm -rf \"$tmpPath\"" EXIT
 
 tmpFile="$tmpPath/$(get_name $finalImageName $finalImageTag)"

pkgs/build-support/fetchbzr/nix-prefetch-bzr

@@ -42,7 +42,7 @@ fi
 # If we don't know the hash or a path with that hash doesn't exist,
 # download the file and add it to the store.
 if test -z "$finalPath"; then
-    tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/bzr-checkout-tmp-XXXXXXXX")"
+    tmpPath="$(mktemp -d "$(realpath "${TMPDIR:-/tmp}/bzr-checkout-tmp-XXXXXXXX")")"
     trap "rm -rf \"$tmpPath\"" EXIT
 
     tmpFile="$tmpPath/$dstFile"

pkgs/build-support/fetchcvs/nix-prefetch-cvs

@@ -20,7 +20,7 @@ fi
 
 
 mkTempDir() {
-    tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/nix-prefetch-cvs-XXXXXXXX")"
+    tmpPath="$(mktemp -d "$(realpath "${TMPDIR:-/tmp}/nix-prefetch-cvs-XXXXXXXX")")"
     trap removeTempDir EXIT
 }
 

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -454,7 +454,7 @@ else
     # download the file and add it to the store.
     if test -z "$finalPath"; then
 
-        tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/git-checkout-tmp-XXXXXXXX")"
+        tmpPath="$(mktemp -d "$(realpath "${TMPDIR:-/tmp}/git-checkout-tmp-XXXXXXXX")")"
         exit_handlers+=(remove_tmpPath)
 
         tmpFile="$tmpPath/$(url_to_name "$url" "$rev")"

pkgs/build-support/fetchhg/nix-prefetch-hg

@@ -43,7 +43,7 @@ fi
 # download the file and add it to the store.
 if [[ -z "$finalPath" ]]; then
 
-    tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/hg-checkout-tmp-XXXXXXXX")"
+    tmpPath="$(mktemp -d "$(realpath "${TMPDIR:-/tmp}/hg-checkout-tmp-XXXXXXXX")")"
     cleanup() { x=$?; rm -rf "$tmpPath"; exit $x; }; trap cleanup EXIT
 
     tmpArchive="$tmpPath/hg-archive"

pkgs/build-support/fetchsvn/nix-prefetch-svn

@@ -41,7 +41,7 @@ fi
 # If we don't know the hash or a path with that hash doesn't exist,
 # download the file and add it to the store.
 if test -z "$finalPath"; then
-    tmpPath="$(mktemp -d "${TMPDIR:-/tmp}/svn-checkout-tmp-XXXXXXXX")"
+    tmpPath="$(mktemp -d "$(realpath "${TMPDIR:-/tmp}/svn-checkout-tmp-XXXXXXXX")")"
     trap "rm -rf \"$tmpPath\"" EXIT
 
     tmpFile="$tmpPath/$dstFile"