Make etc/scripts/get-stack.sh respect the -d argument

[decentral1se]

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

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This change now makes the install script use the -d {path} value when checking for existing stack executables. Should close off #3350. Arguably, we should also use realpath for the 'Installing Stack to: ...' message, no?

Here's my test logs (click dropdown):

Before

λ ~/hobby/stack/ master mkdir foobar
λ ~/hobby/stack/ master cd foobar     
λ ~/hobby/stack/foobar/ master less ../etc/scripts/get-stack.sh | sh -s - -d .    
Stack Version 1.5.1 already appears to be installed at:
  /home/lwm/.local/bin/stack
Use 'stack upgrade' or your OS's package manager to upgrade,
or pass '-f' to this script to over-write the existing binary, e.g.:
  curl -sSL https://get.haskellstack.org/ | sh -s - -f

After

λ ~/hobby/stack/ fixup-3350 mkdir foobar
λ ~/hobby/stack/ fixup-3350 cd foobar                                              
λ ~/hobby/stack/foobar/ fixup-3350 less ../etc/scripts/get-stack.sh | sh -s - -d .
... (regular install logs)
λ ~/hobby/stack/foobar/ fixup-3350* less ../etc/scripts/get-stack.sh | sh -s - -d .
Stack Version 1.5.1 already appears to be installed at:
  /home/lwm/hobby/stack/foobar/stack
Use 'stack upgrade' or your OS's package manager to upgrade,
or pass '-f' to this script to over-write the existing binary, e.g.:
  curl -sSL https://get.haskellstack.org/ | sh -s - -f
λ ~/hobby/stack/foobar/ fixup-3350* less ../etc/scripts/get-stack.sh | sh -s - -d . -f
... (regular install logs)
Installing Stack to: ./stack...

-------------------------------------------------------------------------------

Stack has been installed to: ./stack

WARNING: '.' is not on your PATH.

[borsboom]

Looks good, thanks!