overrides: pin to shim-15.6-2

The new shim-15.8-2 seems to not work for secure boot:
coreos/fedora-coreos-tracker#1694

manifest-lock.overrides.aarch64.yaml

@@ -0,0 +1,16 @@
+# This lockfile should be used to pin to a package version (`type: pin`) or to
+# fast-track packages ahead of Bodhi (`type: fast-track`). Fast-tracked
+# packages will automatically be removed once they are in the stable repos.
+#
+# IMPORTANT: YAML comments *will not* be preserved. All `pin` overrides *must*
+# include a URL in the `metadata.reason` key. Overrides of type `fast-track`
+# *should* include a Bodhi update URL in the `metadata.bodhi` key and a URL
+# in the `metadata.reason` key, though it's acceptable to omit a `reason`
+# for FCOS-specific packages (ignition, afterburn, etc.).
+
+packages:
+  shim-aa64:
+    evr: 15.6-2
+    metadata:
+      reason: https://github.com/coreos/fedora-coreos-tracker/issues/1694
+      type: pin

manifest-lock.overrides.x86_64.yaml

@@ -0,0 +1,17 @@
+
+# This lockfile should be used to pin to a package version (`type: pin`) or to
+# fast-track packages ahead of Bodhi (`type: fast-track`). Fast-tracked
+# packages will automatically be removed once they are in the stable repos.
+#
+# IMPORTANT: YAML comments *will not* be preserved. All `pin` overrides *must*
+# include a URL in the `metadata.reason` key. Overrides of type `fast-track`
+# *should* include a Bodhi update URL in the `metadata.bodhi` key and a URL
+# in the `metadata.reason` key, though it's acceptable to omit a `reason`
+# for FCOS-specific packages (ignition, afterburn, etc.).
+
+packages:
+  shim-x64:
+    evr: 15.6-2
+    metadata:
+      reason: https://github.com/coreos/fedora-coreos-tracker/issues/1694
+      type: pin