Fixes bash command error when DESTDIR is a non-path directory

[frshwtr]

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Problem

We found an issue with the install script on MacOS Catalina with bash 3.2 and Ubuntu 18 with bash 4.4, zsh 5.4.2.

When DESTDIR is set to a directory that is not in PATH, CircleCI CLI will successfully install but the script will error out on line 59.

Example output:

➜ DESTDIR=$(pwd)/clitemp ./install.sh
Starting installation.
Installing CircleCI CLI v0.1.7971
Installing to /home/ben/git/frshwtr/circleci-cli/clitemp
An error occured installing the tool.
The contents of the directory /tmp/tmp.HAAnGm47op have been left in place to help to debug the issue.

We suspect this is because command is being executed with no directory context. Meaning any install to a non-path directory will fail, causing the install script to fail.

Testing

We have ran this locally on macOS Catalina with the following cases:

• settingDESTDIR and examining the exit code and status of the installed binary.
• doing the same with the default (/usr/local/bin) set by the script.

All appears to work well in both cases.

[codecov]

Codecov Report

Merging #431 (4d8be28) into master (168582f) will decrease coverage by 1.91%.
The diff coverage is n/a.

❗ Current head 4d8be28 differs from pull request most recent head 330d65b. Consider uploading reports for the commit 330d65b to get more accurate results

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
- Coverage   32.77%   30.86%   -1.92%     
==========================================
  Files          46       26      -20     
  Lines        5415     3201    -2214     
==========================================
- Hits         1775      988     -787     
+ Misses       3387     2119    -1268     
+ Partials      253       94     -159     

Impacted Files	Coverage Δ
filetree/filetree.go	73.72% <0.00%> (-4.54%)	⬇️
cmd/context.go	25.14% <0.00%> (-3.05%)	⬇️
references/references.go	60.60% <0.00%> (-3.04%)	⬇️
git/git.go	81.25% <0.00%> (-2.36%)	⬇️
cmd/update.go	23.76% <0.00%> (-0.38%)	⬇️
cmd/diagnostic.go	17.30% <0.00%> (-0.20%)	⬇️
main.go	0.00% <0.00%> (ø)
cmd/open.go	50.00% <0.00%> (ø)
cmd/check.go	0.00% <0.00%> (ø)
cmd/disabled.go	0.00% <0.00%> (ø)
... and 37 more

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[jvincent42]

Thanks for your contribution! Sorry for the delay in reviewing your work.
If you still want this PR to be merged, can you please rebase it on main and address the comment I've left ?
Thank you for your time.

[jvincent42]

We might want to remove any potential trailing / in DESTDIR and force a / before the binary name.

Suggested change

	command -v "$DESTDIR"circleci
	command -v "${DESTDIR%/}"/circleci