fix: write file to the current folder

[bzoz]

Trying to run:

ncu-ci walk pr --stats=true --cache=true --markdown results.md

will fail with:

Error: ENOENT: no such file or directory, mkdir
    at Object.mkdirSync (fs.js:940:3)
    at exports.writeFile (C:\Users\ja\AppData\Local\nvs\node\14.1.0\x64\node_modules\node-core-utils\lib\file.js:19:8)
    at WalkCommand.serialize (C:\Users\ja\AppData\Local\nvs\node\14.1.0\x64\node_modules\node-core-utils\bin\ncu-ci:229:9)
    at main (C:\Users\ja\AppData\Local\nvs\node\14.1.0\x64\node_modules\node-core-utils\bin\ncu-ci:371:24)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:97:5) {
  errno: -4058,
  syscall: 'mkdir',
  code: 'ENOENT'
}

(at least on Windows).

This fixes this by first checking if there is any folder to create before calling mkdir.

[codecov]

Codecov Report

Merging #434 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   77.13%   77.13%           
=======================================
  Files          21       21           
  Lines        1496     1496           
=======================================
  Hits         1154     1154           
  Misses        342      342           

Impacted Files	Coverage Δ
lib/file.js	78.26% <100.00%> (ø)

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