Fix the variable regexp pattern on web page (#27161) (#27164)

Backport #27161 by @lng2020

same as (https://github.com/go-gitea/gitea/pull/26910)

Co-authored-by: Nanguan Lin <70063547+lng2020@users.noreply.github.com>

templates/shared/variables/variable_list.tmpl

@@ -73,7 +73,7 @@
 					name="name"
 					id="dialog-variable-name"
 					value="{{.name}}"
-					pattern="^[a-zA-Z_][a-zA-Z0-9_]*$"
+					pattern="^(?!GITEA_|GITHUB_)[a-zA-Z_][a-zA-Z0-9_]*$"
 					placeholder="{{.locale.Tr "secrets.creation.name_placeholder"}}"
 				>
 			</div>