Restore rtools40 path

[nealrichardson]

On apache/arrow#12436 I was trying to update our workflows to use v2 but ran into a problem where pacman could not be found (we use the pacman bundled with rtools as part of building C++ dependencies for the R package).

Doing some digging, it looks like 18ab426 inadvertently removed the line that adds rtools to the path for rtools40.

To confirm, I set up a simple workflow here that looks for pacman. It confirms it is working on master but not v2, and that this PR fixes the issue.

[codecov-commenter]

Codecov Report

Merging #504 (a66acd8) into v2-branch (77eeb6b) will not change coverage.
The diff coverage is n/a.

❗ Current head a66acd8 differs from pull request most recent head 4bb164f. Consider uploading reports for the commit 4bb164f to get more accurate results

@@             Coverage Diff             @@
##           v2-branch      #504   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
===========================================
  Files              2         2           
  Lines             10        10           
===========================================
  Hits              10        10           

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[gaborcsardi]

Thanks much!

[gaborcsardi]

This is in v2 now.

[github-actions]

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