stack upgrade: Call getExecutablePath before overwriting ourself

[andersk]

Fixes: #4462

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Tested with

$ stack install
Copying from /home/anders/haskell/stack/.stack-work/install/x86_64-linux-tinfo6/custom-snapshot-for-building-stack-with-ghc-8.2.2-kf0nGp8fCCJ3/8.2.2/bin/curator to /home/anders/.local/bin/curator
Copying from /home/anders/haskell/stack/.stack-work/install/x86_64-linux-tinfo6/custom-snapshot-for-building-stack-with-ghc-8.2.2-kf0nGp8fCCJ3/8.2.2/bin/stack to /home/anders/.local/bin/stack

Copied executables to /home/anders/.local/bin:
- curator
- stack

$ stack upgrade --force-download --verbose
Version 1.10.0, Git revision fce9fe335b902776b683420ea2ee45461b0fd97c (6718 commits) x86_64 hpack-0.31.1
2019-03-26 00:03:26.077371: [info] Current Stack version: 1.10.0, available download version: 1.9.3
2019-03-26 00:03:26.077749: [info] Forcing binary upgrade
2019-03-26 00:03:26.077909: [info] Querying for archive location for platform: linux-x86_64-static
2019-03-26 00:03:26.078131: [info] Downloading from: https://github.com/commercialhaskell/stack/releases/download/v1.9.3/stack-1.9.3-linux-x86_64-static.tar.gz
2019-03-26 00:03:28.390036: [info] Download complete, testing executable
Version 1.9.3, Git revision 40cf7b37526b86d1676da82167ea8758a854953b (6211 commits) x86_64 hpack-0.31.1
2019-03-26 00:03:28.401838: [info] New stack executable available at /home/anders/.local/bin/stack

$ ls ~/.local/bin/stack*
/home/anders/.local/bin/stack

[snoyberg]

Awesome, thank you!