fix: use configuration-revision for current_rev in flake info metric

The nixos_flake_info metric's current_rev label was incorrectly showing
the nixpkgs input revision (from /run/current-system/nixos-version)
instead of the flake's own revision.

Now reads from /run/current-system/configuration-revision which contains
the flake's self.rev when system.configurationRevision is set in the
NixOS configuration.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

collector/flake_test.go

@@ -3,40 +3,13 @@ package collector
 import (
 	"encoding/json"
 	"os"
-	"path/filepath"
 	"testing"
 )
 
-func TestRevisionPattern(t *testing.T) {
-	tests := []struct {
-		version string
-		wantRev string
-	}{
-		{"25.11.20260203.e576e3c", "e576e3c"},
-		{"1994-294a625", "294a625"},
-		{"25.05.20250101.abcdef1234567890", "abcdef1234567890"},
-		{"no-revision-here", ""},
-		{"", ""},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.version, func(t *testing.T) {
-			matches := revisionPattern.FindStringSubmatch(tt.version)
-			var got string
-			if matches != nil {
-				got = matches[1]
-			}
-			if got != tt.wantRev {
-				t.Errorf("revisionPattern.FindStringSubmatch(%q) = %q, want %q", tt.version, got, tt.wantRev)
-			}
-		})
-	}
-}
-
 func TestGetCurrentSystemRevision(t *testing.T) {
-	// Skip if not on NixOS
-	if _, err := os.Stat(nixosVersionPath); os.IsNotExist(err) {
-		t.Skip("not running on NixOS")
+	// Skip if not on NixOS with system.configurationRevision set
+	if _, err := os.Stat(configRevisionPath); os.IsNotExist(err) {
+		t.Skip("not running on NixOS with system.configurationRevision set")
 	}
 
 	rev, err := getCurrentSystemRevision()
@@ -48,50 +21,6 @@ func TestGetCurrentSystemRevision(t *testing.T) {
 	t.Logf("current system revision: %s", rev)
 }
 
-func TestGetCurrentSystemRevisionFromFile(t *testing.T) {
-	// Create a temp file to simulate /run/current-system/nixos-version
-	dir := t.TempDir()
-	versionPath := filepath.Join(dir, "nixos-version")
-
-	tests := []struct {
-		content string
-		wantRev string
-	}{
-		{"25.11.20260203.e576e3c\n", "e576e3c"},
-		{"1994-294a625\n", "294a625"},
-		{"25.05.20250101.abcdef1234567890\n", "abcdef1"},
-		{"no-hash", ""},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.content, func(t *testing.T) {
-			if err := os.WriteFile(versionPath, []byte(tt.content), 0644); err != nil {
-				t.Fatal(err)
-			}
-
-			// We can't easily test the actual function without modifying the constant,
-			// so we test the pattern extraction logic directly
-			version := tt.content
-			if len(version) > 0 && version[len(version)-1] == '\n' {
-				version = version[:len(version)-1]
-			}
-
-			matches := revisionPattern.FindStringSubmatch(version)
-			var rev string
-			if matches != nil {
-				rev = matches[1]
-				if len(rev) > 7 {
-					rev = rev[:7]
-				}
-			}
-
-			if rev != tt.wantRev {
-				t.Errorf("got revision %q, want %q", rev, tt.wantRev)
-			}
-		})
-	}
-}
-
 func TestFlakeLocksUnmarshal(t *testing.T) {
 	jsonData := `{
 		"revision": "abc1234567890",