Support escaping template variables in environment values

[parksb]

What's the problem this PR addresses?

Resolves #6491

$ yarn init -2
$ echo 'CYPRESS_DOWNLOAD_PATH_TEMPLATE="\${endpoint}/\${platform}-\${arch}/cypress.zip"' > .env.yarn
$ yarn node -p 'process.env.CYPRESS_DOWNLOAD_PATH_TEMPLATE'

# AS-IS
Usage Error: Environment variable not found (endpoint)

# TO-BE
${endpoint}/${platform}-${arch}/cypress.zip

How did you fix it?

Support escaping template variables in environment variable values. When a template variable is escaped with a backslash (e.g., \${VAR}), it is now preserved as a literal string ${VAR} instead of being replaced with an actual environment variable value.

I updated the regex to match values starting with a backslash. When a match starts with a backslash, it is treated as an escaped template and returns the value with only the backslash removed (match.slice(1)).

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.