Pin actions/checkout action to df4cb1c

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
actions/checkout	action	pinDigest	→ df4cb1c

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[yegor256]

@rultor please, try to merge, since 11 checks have passed

[rultor]

@rultor please, try to merge, since 11 checks have passed

@yegor256 OK, I'll try to merge now. You can check the progress of the merge here.

[rultor]

@rultor please, try to merge, since 11 checks have passed

@yegor256 Done! FYI, the full log is here (took me 4min).

[0crat]

@yegor256 Thanks for the review! You've earned +6 points for this: +18 as a basis; -8 for absolutely no comments posted; -4 for too few (4) hits-of-code. Your running score is +654; don't forget to check your Zerocracy account too).