Set PREFLIGHT env var on preflight builds, keep BUILDKITE_PREFLIGHT as deprecated (#761)

Preflight builds now set PREFLIGHT=true as the canonical env var.
BUILDKITE_PREFLIGHT=true is kept for backwards compatibility but
marked as deprecated, giving consumers time to migrate.

Author: pda

cmd/preflight/preflight.go

@@ -120,7 +120,8 @@ func (c *PreflightCmd) Run(kongCtx *kong.Context, globals cli.GlobalFlags) error
 		Commit:  result.Commit,
 		Branch:  result.Branch,
 		Env: map[string]string{
-			"BUILDKITE_PREFLIGHT": "true",
+			"PREFLIGHT":           "true",
+			"BUILDKITE_PREFLIGHT": "true", // deprecated
 		},
 	})
 	if err != nil {

cmd/preflight/preflight_test.go

@@ -122,8 +122,11 @@ func TestPreflightCmd_Run(t *testing.T) {
 		if !strings.HasPrefix(gotReq.Message, "Preflight ") {
 			t.Errorf("expected message starting with 'Preflight ', got %q", gotReq.Message)
 		}
+		if gotReq.Env["PREFLIGHT"] != "true" {
+			t.Errorf("expected PREFLIGHT=true, got %#v", gotReq.Env)
+		}
 		if gotReq.Env["BUILDKITE_PREFLIGHT"] != "true" {
-			t.Errorf("expected BUILDKITE_PREFLIGHT=true, got %#v", gotReq.Env)
+			t.Errorf("expected BUILDKITE_PREFLIGHT=true (deprecated), got %#v", gotReq.Env)
 		}
 	})