cloud_tests: fix the Impish release name

[paride]

Proposed Commit Message

cloud_tests: fix the Impish release name

Commit f5a2449 introduced Impish but left the release name set to
'hirsute'.

Additional Context

Spotted by our CI.

Test Steps

On main:

$ tox -e citest -- run "--os-name=impish" \
  --platform=nocloud-kvm --preserve-data --data-dir=results \
  --verbose "--deb=DAILY_DEB.deb"

# Verbose output, then test failures like:

AssertionError: 'impish' != 'hirsute'
- impish
+ hirsute

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly