fix: ignore global prefix if --prefix is used

[wraithgar]

When --prefix is used, both the local and global prefix values are set
to be identical. This is functionally broken because their directory
structures are inherently different (for instance, in posix the tree is
in lib/node_modules in the global prefix).

This commit makes npm exec ignore the global folders if it detects both
local and global prefix are identical.

Closes #5268

[wraithgar]

before:

after:

[npm-cli-bot]

found 3 benchmarks with statistically significant performance improvements

• app-large: no-cache
• app-medium: 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	39.479 ±1.80	21.124 ±0.25	18.846 ±0.04	21.360 ±1.13	3.219 ±0.00	3.280 ±0.04	2.721 ±0.06	12.578 ±0.06	2.619 ±0.11	3.736 ±0.03
#5291	43.576 ±1.12	20.619 ±0.35	17.459 ±0.40	20.607 ±1.13	3.021 ±0.07	3.091 ±0.04	2.393 ±0.01	12.232 ±0.15	2.517 ±0.00	4.024 ±0.15

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	29.638 ±0.55	15.593 ±0.08	14.048 ±0.27	14.623 ±1.05	2.947 ±0.01	2.987 ±0.12	2.611 ±0.02	9.406 ±0.02	2.545 ±0.02	3.490 ±0.14
#5291	29.564 ±0.41	15.723 ±0.00	13.616 ±0.09	14.288 ±0.48	2.859 ±0.05	2.952 ±0.18	2.594 ±0.00	8.885 ±0.20	2.285 ±0.06	3.028 ±0.03

[nlf]

seems like a reasonable workaround to me, and the current behavior is absolutely wrong, so 👍