ci: fix benchmark

[wellwelwel]

Tis PR ensures that the benchmark runs on the master branch. This makes benchmarks
executed in PRs compare directly against HEAD.

Also, the 150% margin for triggering an alert has a compounding degradation problem:

1. If a benchmark detects 149%, it doesn't notify and the test passes normally.
2. If the next benchmark also has a 149% loss, that loss is on top of the "new 100%", resulting in ~222% (1.49 × 1.49) accumulated performance loss between two distinct PRs, for example.

The margin was reduced to 110% (for now), limiting this silent drift.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.21%. Comparing base (1952441) to head (8af3569).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4120   +/-   ##
=======================================
  Coverage   90.21%   90.21%           
=======================================
  Files          86       86           
  Lines       13740    13740           
  Branches     1660     1660           
=======================================
  Hits        12396    12396           
  Misses       1344     1344           

Flag	Coverage Δ
compression-0	89.44% <ø> (ø)
compression-1	90.19% <ø> (ø)
static-parser-0	87.81% <ø> (ø)
static-parser-1	88.57% <ø> (ø)
tls-0	89.62% <ø> (ø)
tls-1	89.99% <ø> (ø)

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