nixos-rebuild: Fix repl with a relative flake path

Most nixos-rebuild commands support a relative flake path, however
`repl` uses `builtins.getFlake` and that requires an absolute path. So
ensure we pass an absolute path to it, providing UX consistency. Only do
so when the path exists in order to passthrough URLs as-is.

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -559,11 +559,16 @@ if [ "$action" = repl ]; then
         blue="$(echo -e '\033[34;1m')"
         attention="$(echo -e '\033[35;1m')"
         reset="$(echo -e '\033[0m')"
+        if [[ -e $flake ]]; then
+            flakePath=$(realpath "$flake")
+        else
+            flakePath=$flake
+        fi
         # This nix repl invocation is impure, because usually the flakeref is.
         # For a solution that preserves the motd and custom scope, we need
         # something like https://github.com/NixOS/nix/issues/8679.
         exec nix repl --impure --expr "
-          let flake = builtins.getFlake ''$flake'';
+          let flake = builtins.getFlake ''$flakePath'';
               configuration = flake.$flakeAttr;
               motd = ''
                 $d{$q\n$q}

pkgs/os-specific/linux/nixos-rebuild/test/repl.nix

@@ -113,7 +113,7 @@ runCommand "test-nixos-rebuild-repl" {
 
   # cat -n ~/flake.nix
 
-  expect ${writeText "test-nixos-rebuild-repl-expect" ''
+  expect ${writeText "test-nixos-rebuild-repl-absolute-path-expect" ''
     ${expectSetup}
     spawn sh -c "nixos-rebuild repl --fast --flake path:\$HOME#testconf"
 
@@ -138,6 +138,19 @@ runCommand "test-nixos-rebuild-repl" {
     send "lib?nixos\n"
     expect_simple "true"
   ''}
+
+  pushd "$HOME"
+  expect ${writeText "test-nixos-rebuild-repl-relative-path-expect" ''
+    ${expectSetup}
+    spawn sh -c "nixos-rebuild repl --fast --flake .#testconf"
+
+    expect_simple "nix-repl>"
+
+    send "config.networking.hostName\n"
+    expect_simple "itsme"
+  ''}
+  popd
+
   echo
 
   #########