Do not uselessly precompile in Downstream CI

Previously, it would precompile the dep after installing, and then launch a new
test process with different settings and precompile again.
Disable the first precompilation.

Author: jakobnissen

.github/workflows/Downstream.yml

@@ -42,7 +42,7 @@ jobs:
           repository: ${{ matrix.package.user }}/${{ matrix.package.repo }}
           path: downstream
       - name: Load this and run the downstream tests
-        shell: julia --color=yes --project=downstream {0}
+        shell: JULIA_PKG_PRECOMPILE_AUTO=0 julia --color=yes --project=downstream {0}
         run: |
           using Pkg
           try