fix: run postpack after tarball is written

[nlf]

The version bump commit for libnpmpack still needs to take place, but it felt appropriate to make all of these changes as part of this pull request so as to not break things while it's in flight.

Closes #2220

[wraithgar]

Moving the write functionality into libnpmpack feels like a feat: for libnpmpack

[npm-robot]

found 20 benchmarks with statistically significant performance improvements

• app-large: clean, lock-only, cache-only, cache-only:peer-deps, modules-only, no-lock, no-cache, no-modules, no-clean, no-clean:audit
• app-medium: clean, lock-only, cache-only, cache-only:peer-deps, modules-only, no-lock, no-cache, no-modules, no-clean, no-clean:audit

timing results

app-large	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	47.369 ±0.79	29.453 ±0.82	35.381 ±26.23	19.747 ±0.75	2.850 ±0.02	2.663 ±0.05	2.259 ±0.23	11.250 ±0.19	2.203 ±0.09	2.967 ±0.03
#4389	0.377 ±0.01	0.355 ±0.00	0.365 ±0.00	0.355 ±0.00	0.366 ±0.02	0.353 ±0.00	0.353 ±0.00	0.351 ±0.00	0.355 ±0.00	0.356 ±0.00

app-medium	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	35.290 ±2.19	21.516 ±0.03	11.542 ±0.03	13.264 ±0.24	2.309 ±0.02	2.309 ±0.01	2.091 ±0.00	7.936 ±0.26	1.950 ±0.01	2.738 ±0.07
#4389	0.364 ±0.00	0.377 ±0.02	0.401 ±0.00	0.357 ±0.00	0.381 ±0.01	0.374 ±0.02	0.367 ±0.01	0.353 ±0.00	0.356 ±0.00	0.362 ±0.00

[nlf]

Moving the write functionality into libnpmpack feels like a feat: for libnpmpack

fair point! i can change that commit