Security advisory for ssri transitive dependency

There's a new security advisory for the `ssri` package: https://github.com/advisories/GHSA-vx3p-948g-6vhq

The dependency tree looks like:
```sh
serverless-cloudflare-workers@1.2.0
└─┬ webpack@4.46.0
└─┬ terser-webpack-plugin@1.4.5
└─┬ cacache@12.0.4
└── ssri@6.0.1
```