tests: Pass --legacy-peer-deps flag to work around npm v7 bug

This flag is unknown to npm v6, but npm v6 silently ignores unknown
flags.

.github/workflows/frontend-tests.yml

@@ -62,8 +62,10 @@ jobs:
       run: src/tests/frontend/travis/sauce_tunnel.sh
 
     - name: Install Etherpad plugins
+      # The --legacy-peer-deps flag is required to work around a bug in npm v7:
+      # https://github.com/npm/cli/issues/2199
       run: >
-        npm install --no-save
+        npm install --no-save --legacy-peer-deps
         ep_align
         ep_author_hover
         ep_cursortrace