Bump MSRV to 1.91.0

[flip1995]

In preparation of the opendal bump to version 0.58.1. opendal 0.58.1 has an MSRV of 1.91.0, so sccache needs to match this in order to bump the opendal dependency.

cc #2715

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.03553% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.91%. Comparing base (e9b15a3) to head (84f8791).

Files with missing lines	Patch %	Lines
src/jobserver.rs	0.00%	20 Missing ⚠️
src/compiler/rust.rs	36.00%	16 Missing ⚠️
src/compiler/c.rs	84.37%	10 Missing ⚠️
src/compiler/args.rs	71.42%	4 Missing ⚠️
src/compiler/nvcc.rs	78.94%	4 Missing ⚠️
src/commands.rs	40.00%	3 Missing ⚠️
src/server.rs	33.33%	2 Missing ⚠️
src/compiler/compiler.rs	75.00%	1 Missing ⚠️
src/compiler/msvc.rs	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2793      +/-   ##
==========================================
+ Coverage   71.15%   72.91%   +1.75%     
==========================================
  Files          72       72              
  Lines       40677    37275    -3402     
==========================================
- Hits        28943    27178    -1765     
+ Misses      11734    10097    -1637     

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[Xuanwo]

Thank you! Let's go!