fix(keep_alive): adjust permissions to read content

[Guillaumebeuzeboc]

Defining a permission seems to overwrite the default value of other permission (see actions/checkout#254 and actions/checkout#445)

This is causing issues with private repo:

Before: https://github.com/canonical/ros_snapd/actions/runs/12866431569

Fetching the repository
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +1623941631cbdf9992798da3b3e71cacb6e48e37:refs/remotes/origin/jazzy
  remote: Repository not found.
  Error: fatal: repository 'https://github.com/canonical/ros_snapd/' not found
  The process '/usr/bin/git' failed with exit code 128
  Waiting 13 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +1623941631cbdf9992798da3b3e71cacb6e48e37:refs/remotes/origin/jazzy
  remote: Repository not found.
  Error: fatal: repository 'https://github.com/canonical/ros_snapd/' not found
  The process '/usr/bin/git' failed with exit code 128
  Waiting 19 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +1623941631cbdf9992798da3b3e71cacb6e48e37:refs/remotes/origin/jazzy
  remote: Repository not found.
  Error: fatal: repository 'https://github.com/canonical/ros_snapd/' not found
  Error: The process '/usr/bin/git' failed with exit code 128

After: https://github.com/canonical/ros_snapd/actions/runs/12866742457