build: use npm 7 in github actions

Author: MrWolfZ

.github/workflows/main.yml

@@ -18,6 +18,7 @@ jobs:
         with:
           node-version: 14.x
 
+      - run: npm i -g npm@7
       - run: npm ci
       - run: npm run build
 
@@ -30,6 +31,7 @@ jobs:
         with:
           node-version: 14.x
 
+      - run: npm i -g npm@7
       - run: npm ci
       - run: npm run lint
 
@@ -42,6 +44,7 @@ jobs:
         with:
           node-version: 14.x
 
+      - run: npm i -g npm@7
       - run: npm ci
       - run: npm run test-no-progress
       - run: npm run report-coverage
@@ -55,5 +58,6 @@ jobs:
         with:
           node-version: 14.x
 
+      - run: npm i -g npm@7
       - run: npm ci
       - run: npm run tsd