[Bug] : cssnano v5 breaks on yarn/npm with --no-bin-links due to postinstall step for opencollective

**Describe the bug**

cssnano v5 added forced dependency on `opencollective-postinstall` with `postinstall` which breaks on yarn/npm with `--no-bin-links`.



**To Reproduce**

```

$ yarn --frozen-lockfile --non-interactive --no-bin-links --production

yarn install v1.22.5

[1/5] Validating package.json...

[2/5] Resolving packages...

[3/5] Fetching packages...

info fsevents@1.2.13: The platform "linux" is incompatible with this module.

info "fsevents@1.2.13" is an optional dependency and failed compatibility check. Excluding it from installation.

[4/5] Linking dependencies...

[5/5] Building fresh packages...

info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.

error /drone/src/node_modules/cssnano: Command failed.

Exit code: 127

Command: opencollective-postinstall

Arguments: 

Directory: /drone/src/node_modules/cssnano

Output:

/bin/sh: opencollective-postinstall: not found

```



**Expected behavior**

No issue during install in all environments supported by `cssnano`



**Additional context**

Generally acceptable solution here would be to use `node ./node_modules/opencollective-postinstall/index.js`.



`--no-bin-links` is a very useful option on filesystems where creating symlinks is not possible.



In addition note that the build failed on CI here despite the fact that `opencollective-postinstall` should have check for `process.env.CI`.

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