CI: test against MSRV

[msrd0]

Description

• Bump the MSRV to 1.45 so that all dependencies compile
• Test against the MSRV
• Other small improvements: Use the same github action to download rust and don't run actions/checkout multiple times

Checklist

• N/A Formatted code using cargo fmt --all
• N/A Linted code using clippy cargo clippy --all-targets --all-features -- -D warnings
• ✅ Updated README.md using cargo readme -r influxdb -t ../README.tpl > README.md
• ✅ Reviewed the diff. Did you leave any print statements or unnecessary comments?
• N/A Any unfinished work that warrants a separate issue captured in an issue with a TODO code comment

[Empty2k12]

Very useful change! Thanks a lot!

I'm always happy when best practices are implemented into the project.