libstore: Use boost::regex for GC root discovery (backport #13142)

[mergify]

Motivation

As it turns out using std::regex is actually the bottleneck for root discovery. Just substituting std:: -> boost:: makes root discovery twice as fast (3x if counting only userspace time).

Some rather ad-hoc measurements to motivate the switch:

(On master)

nix build github:nixos/nix/1e822bd4149a8bce1da81ee2ad9404986b07914c#nix-cli --out-link result-1e822bd4149a8bce1da81ee2ad9404986b07914c
taskset -c 2,3 hyperfine "result-1e822bd4149a8bce1da81ee2ad9404986b07914c/bin/nix store gc --dry-run --max 0"
Benchmark 1: result-1e822bd4149a8bce1da81ee2ad9404986b07914c/bin/nix store gc --dry-run --max 0
  Time (mean ± σ):     481.6 ms ±   3.9 ms    [User: 336.2 ms, System: 142.0 ms]
  Range (min … max):   474.6 ms … 487.7 ms    10 runs

(After this patch)

taskset -c 2,3 hyperfine "result/bin/nix store gc --dry-run --max 0"
Benchmark 1: result/bin/nix store gc --dry-run --max 0
  Time (mean ± σ):     254.7 ms ±   9.7 ms    [User: 111.1 ms, System: 141.3 ms]
  Range (min … max):   246.5 ms … 281.3 ms    10 runs

boost::regex is a drop-in replacement for std::regex, but much faster. Doing a simple before/after comparison doesn't surface any change in behavior:

result/bin/nix store gc --dry-run -vvvvv --max 0 |& grep "got additional" | wc -l
result-1e822bd4149a8bce1da81ee2ad9404986b07914c/bin/nix store gc --dry-run -vvvvv --max 0 |& grep "got additional" | wc -l

Note

This really doesn't make a dent in the actual runtime of GC itself.

Context

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This is an automatic backport of pull request #13142 done by [Mergify](https://mergify.com).