Setting file backend to the default for RStudio Server [Pro]

[nwstephens]

This pull request sets the file backend by default on Linux servers where the secret service is not running.

On RStudio Server and RStudio Server Pro, keyring defaults to the env backend today by default, because the secret service is designed to run on desktops, not servers (see #53, #72, and #77). However, the file backend provides a superior experience to the env backend on the server. Users can store encrypted secrets to disk with the file backend, whereas the env backend only holds passwords for the life of the R session. The file backend throws no warnings, whereas the env backend throws warnings.

[codecov-commenter]

Codecov Report

Merging #95 (697b2f0) into master (7397e35) will increase coverage by 1.52%.
The diff coverage is 100.00%.

❗ Current head 697b2f0 differs from pull request most recent head 098e09a. Consider uploading reports for the commit 098e09a to get more accurate results

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   31.52%   33.05%   +1.52%     
==========================================
  Files          14       14              
  Lines        1180     1186       +6     
==========================================
+ Hits          372      392      +20     
+ Misses        808      794      -14     

Impacted Files	Coverage Δ
R/backend-file.R	92.20% <100.00%> (+4.37%)	⬆️
R/default_backend.R	81.81% <100.00%> (-5.28%)	⬇️
R/api.R	35.29% <0.00%> (+5.88%)	⬆️
R/assertions.R	100.00% <0.00%> (+12.50%)	⬆️

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[gaborcsardi]

Thank you!