Alias quoted string handling not working as expected

[accuser]

Required information

• Distribution: Ubuntu Server
• Distribution version: 24.04.1
• The output of "incus info" or if that fails:
  • Kernel version: 6.8.0-45-generic
  • Incus version: 6.0.0
  • Storage backend in use: ZFS

Issue description

I wanted to add an incus alias to automatically update and upgrade packages in instances, but the alias failed when I tried to use it due to an unterminated quoted string.

• sh -c 'apt update && apt upgrade' in the instance works as expected.
• incus exec test -- sh -c 'apt update && apt upgrade' in the host works as expected.
• incus alias add up "exec @ARGS@ -- sh -c 'apt update && apt upgrade'" in the host works as expected.
• incus up test-instance does not work: unterminated quoted string (see steps to reproduce).

I can work around this by adding individual aliases for apt update and apt upgrade, but it would be nice to be able to do this with a single alias.

Steps to reproduce

$ incus launch images:ubuntu/24.04 test-instance  --storage default
$ incus alias add up "exec @ARGS@ -- sh -c 'apt update && apt upgrade'"
$ incus alias list
+------------+--------------------------------------------------+
|   ALIAS    |                      TARGET                      |
+------------+--------------------------------------------------+
| up         | exec @ARGS@ -- sh -c 'apt update && apt upgrade' |
+------------+--------------------------------------------------+
$ incus up test-instance
update: 1: Syntax error: Unterminated quoted string