workflows: Fix the spec version check

I removed all instances of "pip install -e ." from our scripts
in 4e889e7 since installing python-tuf is no longer needed (PWD
is in python import paths already).

This is a different case though since here we don't install dependencies
separately and importing python-tuf still requires securesystemslib:
Let's install the dependencies.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

.github/workflows/specification-version-check.yml

@@ -20,6 +20,7 @@ jobs:
           python-version: "3.x"
       - id: get-version
         run: |
+          python3 -m pip install -r requirements/pinned.txt
           script="from tuf.api.metadata import SPECIFICATION_VERSION; \
                   print(f\"v{'.'.join(SPECIFICATION_VERSION)}\")"
           ver=$(python3 -c "$script")