fix(instance): root_volume boot value read and update

[Codelax]

Fix #1303

[codecov-commenter]

Codecov Report

Merging #1372 (8e9b8ca) into master (f9f55ab) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 8e9b8ca differs from pull request most recent head 2158c65. Consider uploading reports for the commit 2158c65 to get more accurate results

@@            Coverage Diff             @@
##           master    #1372      +/-   ##
==========================================
- Coverage   66.69%   66.68%   -0.02%     
==========================================
  Files         122      122              
  Lines       19501    19503       +2     
==========================================
- Hits        13007    13006       -1     
- Misses       5357     5359       +2     
- Partials     1137     1138       +1     

Impacted Files	Coverage Δ
scaleway/resource_instance_server.go	72.91% <100.00%> (+0.06%)	⬆️
scaleway/retryable_transport.go	70.00% <0.00%> (-10.00%)	⬇️

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