Fix problems due to differing prefixes in S3 stores

[noamross]

This PR fixes an issue where the paths to objects in S3 could differ due to different prefix approaches across targets versions (ropensci/targets#1123). After downloading a target from S3 or elsewhere, we don't assume it is under a proper store path. Rather, on reading we create a temporary store (tempfile/objects/downloaded_file), which is actually a symlink to the object in the cache. So the path to the object is now irrelevant.

@emmamendelsohn, since you have git2r installed already, try this with remotes::install_github(ecohealthalliance/relic#11)

[codecov-commenter]

Codecov Report

Attention: 26 lines in your changes are missing coverage. Please review.

Comparison is base (3c98c2c) 25.00% compared to head (02d502c) 24.33%.

❗ Current head 02d502c differs from pull request most recent head d021db7. Consider uploading reports for the commit d021db7 to get more accurate results

Files	Patch %	Lines
R/tar_read_version.R	0.00%	26 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
- Coverage   25.00%   24.33%   -0.67%     
==========================================
  Files          15       15              
  Lines         404      415      +11     
==========================================
  Hits          101      101              
- Misses        303      314      +11     

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[emmamendelsohn]

Awesome, works now