This reverts commit 5631170.

It fixes a warning when using Ruby 3.0, but brings about another one
when using Ruby 2.5, which is what ci uses.  From the logs of that
commit's workflow run[1]:

    +style-check | RUBY=2.5
    +style-check | --> RUN script/fmt
    +style-check | RuboCop 1.12.1
    +style-check | The following cops were added to RuboCop, but are not configured. Please set Enabled to either `true` or `false` in your `.rubocop.yml` file.

    +style-check | Please also note that can also opt-in to new cops by default by adding this to your config:
    +style-check |   AllCops:
    +style-check |     NewCops: enable
    +style-check | Minitest/AssertWithExpectedArgument: # (new in 0.11)
    +style-check |   Enabled: true
    +style-check | Performance/MapCompact: # (new in 1.11)
    +style-check |   Enabled: true
    +style-check | Performance/RedundantEqualityComparisonBlock: # (new in 1.10)
    +style-check |   Enabled: true
    +style-check | Performance/RedundantSplitRegexpArgument: # (new in 1.10)
    +style-check |   Enabled: true
    +style-check | For more information: https://docs.rubocop.org/rubocop/versioning.html
    +style-check | Inspecting 134 files
    +style-check | .C....................................................................................................................................

    +style-check | Offenses:

    +style-check | lib/jekyll/cache.rb:170:23: C: Security/MarshalLoad: Avoid using Marshal.load.
    +style-check |       value = Marshal.load(cached_file)
    +style-check |                       ^^^^

    +style-check | 134 files inspected, 1 offense detected

    +style-check | Try running `script/fmt -a` to automatically fix errors
    +style-check |
    +style-check | Command /bin/sh -c script/fmt failed with exit code 1
    +style-check |
    +style-check | ERROR: Command exited with non-zero code: RUN script/fmt

[1] https://github.com/jekyll/jekyll/runs/2586877102