Fix CI on MSRV check

[JohnTitor]

For instance: https://github.com/actix/actix-net/runs/940187979

This is because we use 1.39.0 for installing cargo-cache, it should be resolved by using the latest stable instead.

[codecov]

Codecov Report

Merging #171 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
+ Coverage   60.41%   60.45%   +0.04%     
==========================================
  Files          73       73              
  Lines        4785     4785              
==========================================
+ Hits         2891     2893       +2     
+ Misses       1894     1892       -2     

Impacted Files	Coverage Δ
actix-connect/tests/test_connect.rs	90.76% <0.00%> (+3.07%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 235a76d...d692ffd. Read the comment docs.