ci: build both debug and non-debug builds, add secondary cache

This runs `test //... -c dbg` in addition to `test //...`. Because switching to-and-from the debug configuration causes the local cache to get thrashed, we add a secondary cache via --disk_cache. This should, at some point, be replaced with a proper remote cache instead.

We also drive-by fix a debug build issue.

Fixes T883.

Test Plan: This should test more things in CI now.

Bug: T883

X-Origin-Diff: phab/D688
GitOrigin-RevId: 9c35c4737d6b205a6bc74d50665c37535ac1d5ba
diff --git a/metropolis/node/core/delve_enabled.go b/metropolis/node/core/delve_enabled.go
index e6ecc2b..dc17f59 100644
--- a/metropolis/node/core/delve_enabled.go
+++ b/metropolis/node/core/delve_enabled.go
@@ -21,7 +21,7 @@
 	"fmt"
 	"os/exec"
 
-	"source.monogon.dev/metropolis/node/"
+	"source.monogon.dev/metropolis/node"
 	"source.monogon.dev/metropolis/node/core/network"
 )