Leave a hint at what to do if the temp dir is not writable in installer

[bwoebi]

Description

It's not always obvious to the user how to work around a permission denied error here.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.90%. Comparing base (c69df0a) to head (c9714cb).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3015      +/-   ##
============================================
+ Coverage     73.88%   73.90%   +0.01%     
  Complexity     2750     2750              
============================================
  Files           111      111              
  Lines         10900    10900              
============================================
+ Hits           8054     8056       +2     
+ Misses         2846     2844       -2     

Flag	Coverage Δ
tracer-php	73.90% <ø> (+0.01%)	⬆️

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see 1 file with indirect coverage changes

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