Refactored webui* rules

* part of nicolargo#2906
* output is unchanged.

```
$ git log --oneline -n1 --pretty=short
commit f2f97e2f (HEAD -> makefile)
Author: Ariel Otilibili <[email protected]>

    Refactored `webui*` rules

$ make webui -n >> /tmp/makefile; echo $?
0

$ make webui-audit -n >> /tmp/makefile; echo $?
0

$ make webui-audit-fix -n >> /tmp/makefile; echo $?
0

$ git switch develop
Switched to branch 'develop'
Your branch is up to date with 'origin/develop'.

$ git log --oneline -n1 --pretty=short
commit 24c8725 (HEAD -> develop, origin/develop, origin/HEAD)
Merge: 42af55b fa7fc5b
Author: Nicolas Hennion <[email protected]>

    Merge pull request nicolargo#2941 from ariel-anieli/refactor-alert

$ make webui -n >> /tmp/develop; echo $?
0

$ make webui-audit -n >> /tmp/develop; echo $?
0

$ make webui-audit-fix -n >> /tmp/develop; echo $?
0

$ diff /tmp/develop /tmp/makefile; echo $?
0
```

Signed-off-by: Ariel Otilibili <[email protected]>

Author: ariel-anieli

Makefile

@@ -203,14 +203,16 @@ install: ## Open a Web Browser to the installation procedure
 # Follow ./glances/outputs/static/README.md for more information
 # ===================================================================
 
+webui webui%: DIR = glances/outputs/static/
+
 webui: ## Build the Web UI
-	cd glances/outputs/static/ && npm ci && npm run build
+	cd $(DIR) && npm ci && npm run build
 
 webui-audit: ## Audit the Web UI
-	cd glances/outputs/static/ && npm audit
+	cd $(DIR) && npm audit
 
 webui-audit-fix: ## Fix audit the Web UI
-	cd glances/outputs/static/ && npm audit fix && npm ci && npm run build
+	cd $(DIR) && npm audit fix && npm ci && npm run build
 
 # ===================================================================
 # Packaging