test: define custom `.npmrc` and snapshot the store

[KSXGitHub]

1. Define a custom .npmrc with store-dir and cache-dir set to a temporary subdirectory in order to avoid conflicts with the main pnpm store.
2. Move current dir of pacquet execution to a subdirectory in a temporary directory.
3. Test snapshot of the store directory on install.

[github-actions[bot]]

Micro-Benchmark Results

Linux

group                          main                                   pr
-----                          ----                                   --
tarball/download_dependency    1.09     10.9±5.74ms   398.7 KB/sec    1.00     10.0±3.97ms   436.0 KB/sec

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d8578e1) 85.93% compared to head (e4e1773) 85.90%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #169 +/- ## ========================================== - Coverage 85.93% 85.90% -0.03% ========================================== Files 53 53 Lines 2680 2718 +38 ========================================== + Hits 2303 2335 +32 - Misses 377 383 +6 ``` | [Files](https://app.codecov.io/gh/pnpm/pacquet/pull/169?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pnpm) | Coverage Δ | | |---|---|---| | [crates/testing-utils/src/bin.rs](https://app.codecov.io/gh/pnpm/pacquet/pull/169?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pnpm#diff-Y3JhdGVzL3Rlc3RpbmctdXRpbHMvc3JjL2Jpbi5ycw==) | `100.00% <100.00%> (ø)` | | | [crates/testing-utils/src/fs.rs](https://app.codecov.io/gh/pnpm/pacquet/pull/169?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pnpm#diff-Y3JhdGVzL3Rlc3RpbmctdXRpbHMvc3JjL2ZzLnJz) | `100.00% <100.00%> (ø)` | | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/pnpm/pacquet/pull/169/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pnpm)

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[github-actions[bot]]

Integrated-Benchmark Report (Linux)

Scenario: Frozen Lockfile

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
pacquet@HEAD	238.3 ± 21.7	207.2	274.8	1.00
pacquet@main	240.0 ± 20.4	205.4	267.7	1.01 ± 0.13