Merge ~philroche/maas-images:feature/shim-signed-rename into maas-images:master

[Philip Roche]

Fix: Migrate to shimx64.efi.signed.latest and shimaa64.efi.signed.latest for shim

Following a rename of the shimx64.efi.signed files to shimx64.efi.signed.latest and
shimx64.efi.signed.previous we need to migrate the new `.latest` file instead.

This fixes the build issue when creating bootloader stream.

```
13:05:31 FileNotFoundError: [Errno 2] No such file or directory: '/tmp/maas-images-onnhjghf/usr/lib/shim/shimx64.efi.signed'
13:05:32 Failed to import the bootloaders
```

[Adam Collard (adam-collard)]

+1, thanks

review: Approve

diff --git a/conf/bootloaders.yaml b/conf/bootloaders.yaml
index 619ed5d..d7d2a67 100644
--- a/conf/bootloaders.yaml
+++ b/conf/bootloaders.yaml
@@ -29,7 +29,7 @@ bootloaders:
    release: focal
    os: grub-efi-signed
    files:
-    - usr/lib/shim/shimx64.efi.signed, bootx64.efi
+    - usr/lib/shim/shimx64.efi.signed.latest, bootx64.efi
     - usr/lib/grub/x86_64-efi-signed/grubnetx64.efi.signed, grubx64.efi
  - firmware-platform: uefi
    packages:
@@ -41,7 +41,7 @@ bootloaders:
    release: focal
    os: grub-efi
    files:
-    - usr/lib/shim/shimaa64.efi.signed, bootaa64.efi
+    - usr/lib/shim/shimaa64.efi.signed.latest, bootaa64.efi
     - usr/lib/grub/arm64-efi-signed/grubnetaa64.efi.signed, grubaa64.efi
  - firmware-platform: open-firmware
    packages: