Installation failure failure - Darwin Mojave 10.14.4

Created by: justanotherdot

Describe the bug Tried running the provided installation script on the README:

$ curl https://raw.githubusercontent.com/haskell/ghcup/master/bootstrap-haskell -sSf | sh

resulting in an installation failure. I tried the recommended ghcup upgrade to no avail. Rerunning the install command directly with verbose gives us:

$ ~/.ghcup/bin/ghcup --cache --verbose install
mkdir -p /Users/$USER/.ghcup
mkdir -p /Users/$USER/.ghcup/bin
mkdir -p /Users/$USER.ghcup/cache
rm /Users/$USER/.ghcup/cache/.available-versions
rm /Users/$USER/.ghcup/cache/.download-urls
cd /Users/$USER/.ghcup/cache
curl --fail -O https://raw.githubusercontent.com/haskell/ghcup/master//.available-versions
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   686  100   686    0     0  10630      0 --:--:-- --:--:-- --:--:-- 10718
cd /Users/$USER/.ghcup/cache
curl --fail -O https://raw.githubusercontent.com/haskell/ghcup/master//.download-urls
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  9237  100  9237    0     0   165k      0 --:--:-- --:--:-- --:--:--  167k
Installing GHC-8.6.3 for Darwin on architecture x86_64
cd /var/folders/dx/v1bjflqj2q741k8fy6hpwglm0000gn/T/tmp.TWp3Ad3g
xz -cd "/Users/$USER/.ghcup/cache/ghc-8.6.3-x86_64-apple-darwin.tar.xz" | tar -xf -
cd ghc-8.6.3
/Users/$USER/.ghcup/bin/ghcup: line 498: cd: ghc-8.6.3: No such file or directory
Failed to install, consider updating this script via: ghcup upgrade

I think the tarball extraction failed, as there is no directory under temporary directory:

$ ls -a /var/folders/dx/v1bjflqj2q741k8fy6hpwglm0000gn/T/tmp.TWp3Ad3g
./      ../

However, when I run the same command manually, the directory shows up:

$ cd /var/folders/dx/v1bjflqj2q741k8fy6hpwglm0000gn/T/tmp.KnXd9jmC
$ xz -cd ~/.ghcup/cache/ghc-8.6.3-x86_64-apple-darwin.tar.xz | tar -xf -
$ ls
ghc-8.6.3/

so it would appear that the tar extraction isn't necessarily failing to produce a directory unless it's something specific to the script. Any ideas?

To Reproduce Steps to reproduce the behavior:

1. curl https://raw.githubusercontent.com/haskell/ghcup/master/bootstrap-haskell -sSf | sh

System information

Script version: 0.0.7

Script variables:
  GHC install location:     /Users/$USER/.ghcup/ghc
  Binary install location:  /Users/$USER/.ghcup/bin
  Tarball cache location:   /Users/$USER/.ghcup/cache
  Downloader:               curl --fail -O <url>
  Script update url:        https://raw.githubusercontent.com/haskell/ghcup/master//ghcup
  GHC download baseurl:     https://downloads.haskell.org/~ghc
  Meta download url         https://raw.githubusercontent.com/haskell/ghcup/master//.download-urls
  Meta download format      1
  Meta version url          https://raw.githubusercontent.com/haskell/ghcup/master//.available-versions
  Meta version format       1

Detected system information:
  Architecture:   x86_64
  Distribution:   Darwin
  Distro alias:   darwin
  Distro version: 18.5.0