NodeJS: NOT able to set PERCY_TOKEN via package script with start-server-and-test

I have been trying really hard to set the PERCY_TOKEN on a local test run according the official guideline from percy and trigger it via yarn.

The guideline says:

$ PERCY_TOKEN=aaabbbcccdddeeefff PERCY_BRANCH=local npm test

My attempt right now looks like this:

package.json

...
"scripts": {
    "start": "react-scripts -r @cypress/instrument-cra start",
    "start:silent": "BROWSER=none yarn start",
    "start:server": "start-server-and-test start:silent http://localhost:3000",
    "build": "react-scripts build",
    "eject": "react-scripts eject",
    "envGenerateExample": "cat .env | sed 's/=.*/=/g' > .env.example",
    "jest:test": "react-scripts test --env=jest-environment-jsdom-sixteen",
    "cy:run": " 'yarn start:server './node_modules/.bin/cypress run'",
    "cy:open": "yarn start:server './node_modules/.bin/cypress open'",
    "cy:ci": "yarn start:server cy:chrome",
    "cy:chrome": "cypress run --browser chrome --record",
    "percy:exec": "yarn percy exec -- cypress run",
    "cy:percy": "yarn start:server percy:exec",
    "percy:local": "PERCY_TOKEN=$(grep 'PERCY_TOKEN.*' .env | sed 's/.*=//'); PERCY_BRANCH=local;",
    "cy:percy:local": "yarn percy:local && yarn start:server percy:exec"
  },
...

$ yarn cy:percy:local

Compiled successfully!

You can now view playground in the browser.

  Local:            http://localhost:3000
  On Your Network:  http://192.168.1.163:3000

Note that the development build is not optimized.
To create a production build, use yarn build.

...

> playground@0.2.0 percy:exec /Users/norfeldt/Repos/playground
> yarn percy exec -- cypress run

warning From Yarn 1.0 onwards, scripts don't require "--" for options to be forwarded. In a future version, any explicit "--" will be forwarded as-is to the scripts.
$ /Users/norfeldt/Repos/playground/node_modules/.bin/percy exec cypress run
 ›   Warning: Skipping visual tests. PERCY_TOKEN was not provided.
...

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