Add missing install with pip

.github/workflows/unix_impl.yml

@@ -167,7 +167,7 @@ jobs:
   verify_pkg_tests:
     name: mamba-content-trust tests
     needs: ["build_shared_unix"]
-    runs-on: ubuntu-latest
+    if: startsWith(inputs.os, 'ubuntu')
     steps:
       - name: Checkout mamba repository
         uses: actions/checkout@v4
@@ -185,8 +185,10 @@ jobs:
       - name: Run tests using conda-content-trust (server side)
         shell: bash -l {0} -euo pipefail -x
         run: |
-          export TEST_MAMBA_EXE="${PWD}/build/micromamba/mamba"
+          export TEST_MAMBA_EXE="${pwd}/build/micromamba/mamba"
           export MAMBA_ROOT_PREFIX="${HOME}/micromamba"
+          unset CONDARC  # Interferes with tests
+          python -m pip install securesystemslib --no-input --no-deps
           cd micromamba/test-server
           ./generate_gpg_keys.sh
           ./testserver_pkg_signing.sh