Add flit-core installation for pip 26+ compatibility

pip 26+ uses flit-core as its build backend (via PEP 517) instead of
setuptools. When installing pip with BP_PIP_VERSION=latest, we need to
provide flit-core in the --find-links directory so pip can build itself.

Changes:
- InstallPip() now downloads flit-core to /tmp/pip alongside pip
- Gracefully handles missing flit-core (cflinuxfs3 with pip 25.2)
- Only logs warning if flit-core unavailable (doesn't fail)

This fixes the error:
  ERROR: Could not find a version that satisfies the requirement
  flit-core<4,>=3.11 (from versions: none)

Note: flit-core is only in manifest for cflinuxfs4/5, which matches
the stacks that use pip 26.1.2 after the previous commit.

Related:
- pip 26.1.2 pyproject.toml requires: flit-core<4,>=3.11
- Manifest has flit-core 3.12.0 for cflinuxfs4/cflinuxfs5

Author: ivanovac

src/python/supply/supply.go

@@ -341,6 +341,13 @@ func (s *Supplier) InstallPip() error {
 		return err
 	}
 
+	// pip 26+ requires flit-core as a build dependency
+	// Install it to the same tempPath so pip can find it during build
+	if err := s.Installer.InstallOnlyVersion("flit-core", tempPath); err != nil {
+		// Log warning but continue - older pip versions don't need flit-core
+		s.Log.Warning("Could not install flit-core (required for pip 26+): %v", err)
+	}
+
 	if err := s.Command.Execute(s.Stager.BuildDir(), indentWriter(os.Stdout), indentWriter(os.Stderr),
 		"python",
 		"-m", "pip",