`process.env.*` in strings should not be replaced

[MikeRomaa]

Describe the problem

Mach will replace both occurrences of process.env.SENTRY_DSN in the following code:

if (process.env.SENTRY_DSN) {
    Sentry.init({ dsn: process.env.SENTRY_DSN });
} else {
    console.warn("`process.env.SENTRY_DSN` is not configured, errors will not be logged in Sentry");
}

This leads to an error that claims:

Expected ")" but found "https"

This is due to the fact that the second occurrence is also replaced, which causes the quotation signs to get clobbered.

Mach should only replace references to the literal process.env variable--not any occurrence of the string in the source code.