ci: use codecov/codecov-action@v1

[h-michael]

https://community.codecov.io/t/http-400-with-version2-1-5-and-github-actions-token-problem/1593/3

[codecov]

Codecov Report

Merging #79 into master will increase coverage by 0.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   56.10%   56.62%   +0.51%     
==========================================
  Files          11       11              
  Lines        2636     2128     -508     
==========================================
- Hits         1479     1205     -274     
+ Misses       1021      789     -232     
+ Partials      136      134       -2     

Impacted Files	Coverage Δ
nvim/plugin/plugin.go	31.50% <0.00%> (-3.86%)	⬇️
msgpack/encode.go	76.74% <0.00%> (-3.55%)	⬇️
msgpack/rpc/endpoint.go	58.08% <0.00%> (-3.17%)	⬇️
msgpack/decode.go	63.79% <0.00%> (-3.05%)	⬇️
msgpack/unpack.go	82.94% <0.00%> (-2.24%)	⬇️
msgpack/common.go	70.78% <0.00%> (-1.95%)	⬇️
nvim/apiimp.go	20.09% <0.00%> (-1.90%)	⬇️
nvim/plugin/main.go	31.11% <0.00%> (-0.14%)	⬇️
msgpack/pack.go	87.57% <0.00%> (-0.06%)	⬇️
nvim/nvim.go	68.85% <0.00%> (+0.10%)	⬆️
... and 1 more

---

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 acbae1a...72cd22e. Read the comment docs.