`swiftly` shouldn't install files prior to approval

[timsneath]

Summary:
Swiftly prompts me to confirm where I'd like to install files prior to installation. But it installs files

Actual:
On a clean macOS machine, I ran the install command, and the following text is printed (emphasis mine):

Welcome to swiftly, the Swift toolchain manager for Linux and macOS!

Please read the following information carefully before proceeding with the installation. If you
wish to customize the steps performed during the installation process, refer to 'swiftly init -h'
for configuration options.

Swiftly installs files into the following locations:

/Users/timsneath/.swiftly - Directory for configuration files
/Users/timsneath/.swiftly/bin - Links to the binaries of the active toolchain
/Users/timsneath/Library/Developer/Toolchains - Directory hosting installed toolchains

Proceed? (Y/n):
n
Error: swiftly installation has been cancelled

Despite selecting n, Swiftly installed files to my home drive:

% ls -lR ~/.swiftly
timsneath@Tims-Loaner-MacBook-Pro containerization % ls -lR ~/.swiftly
total 0
drwxr-xr-x  3 timsneath  staff  96 Jun 12 10:51 bin
drwxr-xr-x  3 timsneath  staff  96 Jun 12 09:57 share

/Users/timsneath/.swiftly/bin:
total 43104
-rwxr-xr-x  1 timsneath  staff  22068688 Jun 12 10:51 swiftly

/Users/timsneath/.swiftly/share:
total 0
drwxr-xr-x  3 timsneath  staff  96 Jun 12 09:57 doc

/Users/timsneath/.swiftly/share/doc:
total 0
drwxr-xr-x  3 timsneath  staff  96 Jun 12 09:57 swiftly

/Users/timsneath/.swiftly/share/doc/swiftly:
total 0
drwxr-xr-x  3 timsneath  staff  96 Jun 12 09:57 license

/Users/timsneath/.swiftly/share/doc/swiftly/license:
total 24
-rw-r--r--@ 1 timsneath  staff  11358 Jun 12 09:57 LICENSE.txt

Expected:
If I say "no" to proceeding, I don't expect swiftly to install any files. Or at minimum, I expect it to clean up any files it's already installed. The clear implication of the text is that it's asking for me to agree that this is the location for Swiftly to be installed, and giving me the opportunity to bail out if I want to install it somewhere else.

[louisunlimited]

Good catch. The reason is probably because the installation script:

curl -O https://download.swift.org/swiftly/darwin/swiftly.pkg && \
installer -pkg swiftly.pkg -target CurrentUserHomeDirectory && \
~/.swiftly/bin/swiftly init --quiet-shell-followup && \
. "${SWIFTLY_HOME_DIR:-$HOME/.swiftly}/env.sh" && \
hash -r

used installer which installs the .pkg file, which creates the swiftly binary and LICENSE files in ~/.swiftly/ way before swiftly is even being invoked.

If we decide to keep this install command we should at least do some clean ups when user selects No during swiftly init, or if we can support some other package managers like brew which would avoid this problem.

cc. @cmcgee1024