Do not uselessly precompile in Downstream CI (#340)

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

@@ -44,6 +44,7 @@ jobs:
       - name: Load this and run the downstream tests
         shell: julia --color=yes --project=downstream {0}
         run: |
+          ENV["JULIA_PKG_PRECOMPILE_AUTO"] = 0
           using Pkg
           try
             # force it to use this PR's version of the package