Fix paths to lint, pin version of flake8.

Results of flake8 vary with major version updates.
Let's pin the version of flake8 to make sure there are no surprises.

Pinning this to 3.7.9, as 3.8 changed behaviour.

Author: Pike

.github/workflows/lint.yml

@@ -22,10 +22,11 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install flake8
+          pip install flake8==3.7.9
       - name: lint
+        working-directory: ./fluent.syntax
         run: |
-          flake8 fluent-syntax/fluent
+          flake8 fluent
 
   runtime:
     name: flake8 runtime
@@ -38,7 +39,8 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install flake8
+          pip install flake8==3.7.9
       - name: lint
+        working-directory: ./fluent.runtime
         run: |
-          flake8 fluent-runtime/fluent
+          flake8 fluent