Fix unsafe evaluation of inputs.use-sudo (#124)

The previous code was (presumably mistakenly) running inputs.use-sudo as
a command, which worked assuming it was true or false (as they ignore
their arguments), but this results in the action executing the value of
inputs.use-sudo as a command.

Signed-off-by: Hal Blackburn <hwtb2@cam.ac.uk>

action.yml

@@ -176,7 +176,7 @@ runs:
         esac
 
         SUDO=
-        if "${{ inputs.use-sudo }}" == "true" && command -v sudo >/dev/null; then
+        if [[ "${{ inputs.use-sudo }}" == "true" ]] && command -v sudo >/dev/null; then
           SUDO=sudo
         fi