Optimize mmctl CI run (#24732)

* Optimize mmctl CI run

- Remove coverage mode. They increase run time by 3x.
- Remove -race mode in PR builds. They increase run time by 2x and memory consumption as well.

```release-note
NONE
```

* caching go.sum path

```release-note
NONE
```

server/Makefile

@@ -64,7 +64,7 @@ GOTESTSUM_JSONFILE ?= gotestsum.json
 
 # mmctl
 MMCTL_BUILD_TAGS =
-MMCTL_TESTFLAGS = -timeout 30m -race
+MMCTL_TESTFLAGS ?= -timeout 30m
 MMCTL_PKG = github.com/mattermost/mattermost/server/v8/cmd/mmctl/commands
 LDFLAGS += -X "$(MMCTL_PKG).gitCommit=$(BUILD_HASH)"
 LDFLAGS += -X "$(MMCTL_PKG).gitTreeState=$(GIT_TREESTATE)"