ci: use correct rust mounts

[morrisonlevi]

Description

The old mount was too generic and clobbered over the new cargo and rustup installations. What's strange is the new location installs to where the old one meant to go, but didn't actually. The CI was then mounting stuff over the top of the intended location. Fun sequence of consequences.

Reviewer checklist

• [ ] Test coverage seems ok.
• [ ] Appropriate labels assigned.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 79.37%. Comparing base (8180fdb) to head (78dec71). :warning: Report is 764 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2627      +/-   ##
============================================
+ Coverage     78.67%   79.37%   +0.69%     
  Complexity     2198     2198              
============================================
  Files           199      199              
  Lines         21955    21955              
============================================
+ Hits          17273    17426     +153     
+ Misses         4682     4529     -153     

Flag	Coverage Δ
tracer-extension	78.52% <ø> (ø)
tracer-php	80.57% <ø> (+1.68%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more. see 7 files with indirect coverage changes

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