npm --version fails during install-tool npm

[rarkins]

Reproduction:

❯ docker run --rm renovate/renovate bash -c "install-tool npm 6.14.15 || true && echo 'Installed npm version is:' && npm --version" 
Installing tool npm v6.14.15
/home/ubuntu/npm/6.14.15/bin/npm -> /home/ubuntu/npm/6.14.15/lib/node_modules/npm/bin/npm-cli.js
/home/ubuntu/npm/6.14.15/bin/npx -> /home/ubuntu/npm/6.14.15/lib/node_modules/npm/bin/npx-cli.js
+ npm@6.14.15
added 437 packages from 891 contributors in 20.191s
npm ERR! It doesn't look like npm is installed.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/ubuntu/.npm/_logs/2021-12-08T09_01_28_815Z-debug.log
Installed npm version is:
6.14.15

[viceice]

fixed in #221