ci: use rancher head version instead of rc version

[juadk]

Fix #720

In the CI, we don't really test latest rancher version but release candidate version.
It could be better to test latest updates, that's why we want to test v2.7-head by default.

Older behavior

When we set rancherVersion to latest/devel, it installed latest rc version.

New behavior

With rancherVersion equal to latest/devel, we install the head version (by default)
With rancherVersion equal to latest/2.7.5-rc3, we install 2.7.5 release candidate 3

PS: The head version is hardcoded but I don't know if we can make it better without adding a new variable, could be improve later.

Verification runs

• With rancherVersion set to latest/2.7.5-rc3 https://github.com/rancher/elemental/actions/runs/5141439434/jobs/9253956631
  I checked the pod image and it was ok:

  containers:
  - args:
    - --http-listen-port=80
    - --https-listen-port=443
    - --add-local=true
    env:
    - name: CATTLE_NAMESPACE
      value: cattle-system
    - name: CATTLE_PEER_SERVICE
      value: rancher
    - name: CATTLE_SERVER_URL
      value: https://84.203.173.34.bc.googleusercontent.com
    - name: CATTLE_BOOTSTRAP_PASSWORD
      value: rancherpassword
    image: rancher/rancher:v2.7.5-rc3

UI-K3s-Rancher_Latest: failed because there is a login issue with v2.7.5-rc3 but the logic is ok.

• With rancherVersion set to latest/devel (default choice)
  https://github.com/rancher/elemental/actions/runs/5142244935/jobs/9255751656

  containers:
  - args:
    - --http-listen-port=80
    - --https-listen-port=443
    - --add-local=true
    env:
    - name: CATTLE_NAMESPACE
      value: cattle-system
    - name: CATTLE_PEER_SERVICE
      value: rancher
    - name: CATTLE_SERVER_URL
      value: https://64.144.192.35.bc.googleusercontent.com
    - name: CATTLE_BOOTSTRAP_PASSWORD
      value: rancherpassword
    image: rancher/rancher:v2.7-head

UI-K3s-Rancher_Latest: we can see that v2.7-head was used.