Use a separate GOCACHE for config and config-next (#7136)

`t.sh` and `tn.sh` now each use a distinct GOCACHE. This causes
back-to-back invocations of the unittests in config and config-next
to not incorrectly skip tests whose behavior changes between
those two environments.

docker-compose.next.yml

@@ -5,3 +5,4 @@ services:
       FAKE_DNS: 10.77.77.77
       BOULDER_CONFIG_DIR: &boulder_config_dir test/config-next
       GOFLAGS: -mod=vendor
+      GOCACHE: /boulder/.gocache/go-build-next

docker-compose.yml

@@ -9,6 +9,7 @@ services:
       # FAKE_DNS: 172.17.0.1
       FAKE_DNS: 10.77.77.77
       BOULDER_CONFIG_DIR: test/config
+      GOCACHE: /boulder/.gocache/go-build
       GOFLAGS: -mod=vendor
       # Forward the parent env's GOEXPERIMENT value into the container.
       GOEXPERIMENT: ${GOEXPERIMENT}

test.sh

@@ -191,6 +191,7 @@ trap "print_outcome" EXIT
 settings="$(cat -- <<-EOM
     RUN:                ${RUN[@]}
     BOULDER_CONFIG_DIR: $BOULDER_CONFIG_DIR
+    GOCACHE:            $(go env GOCACHE)
     UNIT_PACKAGES:      ${UNIT_PACKAGES[@]}
     UNIT_FLAGS:         ${UNIT_FLAGS[@]}
     FILTER:             ${FILTER[@]}