ci/version-info: create temp files in a temp directory

Avoid polluting the repo with temporary `channels.{nix,json,toml}`
files. Write them to a temporary working directory instead.

(cherry picked from commit bc997a2409)

update-scripts/version-info/default.nix

@@ -20,6 +20,10 @@ writeShellApplication {
   };
 
   text = ''
+    work=$(mktemp -d)
+    (
+      cd "$work"
+
       # Download channel info from NixOS/infra
       curl ${channelsURL} | nix eval --file - --json > channels.json
 
@@ -29,15 +33,18 @@ writeShellApplication {
         --argstr system ${stdenv.hostPlatform.system} \
         --arg-from-file channelsJSON channels.json \
         --out-link channels.toml
+    )
 
-    (
+    {
       echo "# DO NOT MODIFY!"
       echo "# This file was generated by ${
         lib.strings.removePrefix (toString ../.. + "/") (toString ./default.nix)
       }"
       cat ${mainInfo}
       echo
-      cat channels.toml
-    ) > version-info.toml
+      cat "$work/channels.toml"
+    } > version-info.toml
+
+    rm -rf "$work"
   '';
 }