fix(ci-run): add fallback for Go version installation in CI script

This patch allows to keep the build working even if the snap version expected
in the juju go mod hasn't be released.
It fallback on the latest version available and use GOTOOLCHAIN to ensure we use the
expected version of Go.

**Updated files:**

- *jobs/ci-run/scripts/package-juju-source.sh*:
  Added a fallback mechanism to install the latest Go version after failing
  to install the specified version. Also included a workaround by exporting `GOTOOLCHAIN`
  to ensure compatibility with the required Go version for Juju.

Author: gfouillet

jobs/ci-run/scripts/package-juju-source.sh

@@ -11,9 +11,24 @@ if [ -z "$GOVERSION" ]; then
     exit 1
 fi
 
+
 export PATH="/snap/bin:$PATH"
 GO_MAJOR_MINOR=$(echo ${GOVERSION} | cut -d '.' -f1,2)
-sudo snap refresh go --channel="${GO_MAJOR_MINOR}/stable" || sudo snap install go --channel="${GO_MAJOR_MINOR}/stable" --classic
+if sudo snap refresh go --channel="${GO_MAJOR_MINOR}/stable" || sudo snap install go --channel="${GO_MAJOR_MINOR}/stable" --classic; then
+   echo "Successfully installed Go ${GO_MAJOR_MINOR}"
+else
+   # If specific version installation fails, install latest available
+   echo "Failed to install Go ${GO_MAJOR_MINOR}, installing latest available version"
+   if sudo snap refresh go --channel=latest/stable || sudo snap install go --channel=latest/stable --classic; then
+       echo "Successfully installed latest Go version: $(go version)"
+       # Export GOTOOLCHAIN to use the expected version
+       # Note: this is a hack which avoid breaking the CI if for some reason snap go is behind the required version for
+       # Juju. It is not something that should happens regularly, and whenever the snap go is available, this hack won't
+       # be used.
+       export GOTOOLCHAIN="go${GOVERSION}.0+auto"
+       echo "Exported GOTOOLCHAIN=${GOTOOLCHAIN}"
+   fi
+fi
 
 # TODO - fix this workaround
 # As we clone the full history (can't shallow clone otherwise queued jobs miss