Run fails on Dependabot PRs

[mkroening]

When configured as

on:
  push:
    paths: 
      - '**/Cargo.toml'
      - '**/Cargo.lock'

the job consistently fails when Dependabot creates the PR. When I open a PR, the job consistently succeeds.

I am not sure if this is related to #28.

Meanwhile, I will disable the check on pushes.