Include input to action conditionaly

I have an action which performs a SBT build with caching. I would like to be able to toggle if restore-keys are use or not (I do not want to use partial cache hits when working in master, but I want them in a feature branch). The action looks like this:

name: "SBT Build"
description: "Build the project"
inputs:
  incremental:
    description: Allow incremental build (uses result from previous runs)
    default: "false"
    required: false
runs:
  using: composite
  steps:
    - name: Cache SBT and dependencies
      uses: actions/cache@v2
      id: sbt-cache
      with:
        path: |
          ~/.cache/coursier/v1
          ~/.sbt
          project/target
          project/project/target
        key: ${{ runner.os }}-sbt-v2-true-${{ hashFiles('*.sbt', 'project/*.sbt', 'project/build.properties') }}
        restore-keys: ${{ runner.os }}-sbt-v2-${{ inputs.incremental }}-

#  .... more steps  follow ...

Note: currently I use a trick of expanding inputs.incremental into the key. When someone pass the incremental: “true”, the restore key will match the cache name. When “false” is used (which is the default), it will match nothing. It should works, but it looks ugly and I would rather skip the restore-keys or when incremental is not passed.

Is there some expresssion or something like that which would allow me to do this?