config.mode and configEnv.mode variables will be inconsistent in some cases.

[hyf0]

Describe the bug

config.mode and configEnv.mode variables will be inconsistent in some cases.

Reproduction

https://github.com/iheyunfei/vite-bug-report-4437/blob/master/vite.config.js

System Info

System:
    OS: macOS 10.15.7
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 300.63 MB / 32.00 GB
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 14.15.0 - ~/.nvm/versions/node/v14.15.0/bin/node
    Yarn: 1.22.10 - ~/.nvm/versions/node/v14.15.0/bin/yarn
    npm: 6.14.8 - ~/.nvm/versions/node/v14.15.0/bin/npm
  Browsers:
    Chrome: 83.0.4103.106
    Edge: 90.0.818.66
    Safari: 13.1.3
  npmPackages:
    vite: ^2.4.4 => 2.4.4

Used Package Manager

yarn

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to https://github.com/vuejs/vue-next instead.
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