tests: use emtpy string instead of boolean value for snapd_deb_from_repo

.github/workflows/spread-tests.yaml

@@ -33,8 +33,7 @@ on:
       spread-snapd-deb-from-repo:
         description: 'If true, will use the snapd package from the repository when possible'
         required: false
-        type: boolean
-        default: true
+        type: string
       spread-experimental-features:
         description: 'Comma-separated list of experimental snapd features to enable with: snap set system "experimental.<feature-name>=true"'
         required: false
@@ -44,7 +43,6 @@ on:
 jobs:
   run-spread:
     env:
-      SPREAD_SNAPD_DEB_FROM_REPO: ${{ inputs.spread-snapd-deb-from-repo }}
       SPREAD_EXPERIMENTAL_FEATURES: ${{ inputs.spread-experimental-features }}
 
     runs-on: ${{ fromJSON(inputs.runs-on) }}
@@ -73,6 +71,10 @@ jobs:
         # spread uses tags as delta reference
         fetch-depth: 0
 
+    - name: set SPREAD_SNAPD_DEB_FROM_REPO=1
+      if: ${{ inputs.spread-snapd-deb-from-repo == 'true' }}
+      run: echo "SPREAD_SNAPD_DEB_FROM_REPO=1" >> $GITHUB_ENV
+
     - name: Get previous attempt
       id: get-previous-attempt
       run: |