Update install-nix-action to v20

[regadas]

Thanks for this project!

Fixes current build issues https://github.com/cmacrae/emacs/actions/runs/5019343020/jobs/8999719648#step:4:15

Error: Action failed with error: Error: Unable to locate executable
file: cachix. Please verify either the file path exists or the file can
be found within a directory specified by the PATH environment variable.
Also check the file mode to verify the file is executable.