ci: update pull request template

[va-an]

Related #235.

Description

I noticed that although we use just, in the PR template we specify cargo, and in some cases cargo clippy won't show warnings (for example, if a PR changes code for a non-default feature), whereas just clippy will show all warnings because --all-features is added for just.

We could simply add --all-features, but I propose going further and using just. And after merging #235, use a single command just pre-push instead of clippy and fmt.

Checklists

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[coveralls]

Pull Request Test Coverage Report for Build 21985678347

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 10.716%

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Totals
Change from base Build 21153868360:	-0.001%
Covered Lines:	268
Relevant Lines:	2501

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💛 - Coveralls