Avoid unreliable assert_cmd::cargo::cargo_bin

[kornelski]

sccache's own tests fail when [build] build-dir is set in Cargo's config. This is because cargo_bin() makes no-longer-valid assumptions about Cargo's build directory layout: assert-rs/assert_cmd#257

Cargo has a built-in env var for the path, so cargo_bin() can be removed. The var is set only at build time, not at test run time, which makes env! necessary.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.02%. Comparing base (d4ae480) to head (eb22c95).

Files with missing lines	Patch %	Lines
tests/helpers/mod.rs	50.00%	1 Missing ⚠️
tests/oauth.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2489      +/-   ##
==========================================
- Coverage   71.03%   71.02%   -0.01%     
==========================================
  Files          64       64              
  Lines       35317    35318       +1     
==========================================
- Hits        25086    25085       -1     
- Misses      10231    10233       +2     

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