chore: Minor rbac memory optimization (#7391)

* test: Add benchmark for static rbac roles
* static roles should only be allocated once
* A unit test that modifies the ast value should not mess with the globals
* Cache subject AST values to avoid reallocating slices

coderd/database/dbauthz/dbauthz.go

@@ -144,7 +144,8 @@ var (
 			},
 		}),
 		Scope: rbac.ScopeAll,
-	}
+	}.WithCachedASTValue()
+
 	subjectAutostart = rbac.Subject{
 		ID: uuid.Nil.String(),
 		Roles: rbac.Roles([]rbac.Role{
@@ -161,7 +162,8 @@ var (
 			},
 		}),
 		Scope: rbac.ScopeAll,
-	}
+	}.WithCachedASTValue()
+
 	subjectSystemRestricted = rbac.Subject{
 		ID: uuid.Nil.String(),
 		Roles: rbac.Roles([]rbac.Role{
@@ -188,7 +190,7 @@ var (
 			},
 		}),
 		Scope: rbac.ScopeAll,
-	}
+	}.WithCachedASTValue()
 )
 
 // AsProvisionerd returns a context with an actor that has permissions required