fix(vitest): set SSR env only when transformMode is ssr (#4083)

Author: sheremet-va

packages/vitest/src/node/plugins/index.ts

@@ -140,7 +140,6 @@ export async function VitestPlugin(options: UserConfig = {}, ctx = new Vitest('t
         // so we are making them truthy
         process.env.PROD ??= PROD ? '1' : ''
         process.env.DEV ??= DEV ? '1' : ''
-        process.env.SSR ??= '1'
 
         for (const name in envs)
           process.env[name] ??= envs[name]

packages/vitest/src/runtime/setup.node.ts

@@ -37,6 +37,10 @@ export async function setupGlobalEnv(config: ResolvedConfig, { environment }: Re
     _require.extensions['.scss'] = () => ({})
     _require.extensions['.sass'] = () => ({})
     _require.extensions['.less'] = () => ({})
+    process.env.SSR = ''
+  }
+  else {
+    process.env.SSR = '1'
   }
 
   installSourcemapsSupport({