Run integration tests in a temporary environment

This avoids making ChainRulesCore a dependency of the downstream package, which
can cause Aqua failures.

Author: JamesWrigley

.github/workflows/IntegrationTest.yml

@@ -35,12 +35,13 @@ jobs:
           repository: ${{ matrix.package.user }}/${{ matrix.package.repo }}
           path: downstream
       - name: Load this and run the downstream tests
-        shell: julia --project=downstream {0}
+        shell: julia --project=@temp {0}
         run: |
           using Pkg
+          Pkg.develop(PackageSpec(path="."))
           try
             # force it to use this PR's version of the package
-            Pkg.develop(PackageSpec(path="."))  # ResolverError with main deps may be thrown here
+            Pkg.develop(PackageSpec(path="downstream"))  # ResolverError with main deps may be thrown here
             Pkg.update()
             Pkg.test()  # ResolverError with test deps may be thrown here (before test process starts)
           catch err