Follow XDG base dir spec in install script

[arockett]

#370

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• Implement: modify install script to use $XDG_BIN_HOME as install dir and default to $HOME/.local/bin if $XDG_BIN_HOME doesn't exist
• Implement: modify sst upgrade command to use the same logic for determining the install dir
• Test: run install script with
  • $XDG_BIN_HOME set and already in $PATH
  • $XDG_BIN_HOME set and not in $PATH
  • $XDG_BIN_HOME not set
• Test: verify sst upgrade works as expected
  • build sst executable and manually install
  • run sst upgrade and verify it uses the $XDG_BIN_HOME dir

[thdxr]

hey we actually overhauled our install script - do you mind fixing conflicts? this is a good fix - we also need to update our sst upgrade command to respect it

[arockett]

Ah yes, I found the place starting @ https://github.com/sst/ion/blob/dev/pkg/global/upgrade.go#L78 to align the sst upgrade command with this change. I'll have time next week Monday to get to this and I'll make sure to pull in the latest install script changes.

[arockett]

Ok this is ready for another review.

I want to call your attention to some details which I added in the commit description about prior sst installs:

An existing install in the ~/.sst/bin dir is not removed during the `sst
upgrade` command nor is the $PATH modified by `sst upgrade`.

When there is an existing install of sst prior to this commit, the
safest upgrade path is to:
1. delete the ~/.sst dir
2. remove ~/.sst/bin from $PATH by editing the right shell cfg file
3. rerun the install script

The most unexpected situation arises when a user has

• installed a version of sst prior to this commit
• added ~/.sst/bin to their PATH
• does not have $XDG_BIN_HOME on their $PATH, and
• runs sst upgrade to get latest version instead of using the install script

In that situation, running sst upgrade will install the latest sst bin including this change at the old ~/.sst/bin location... still works, then next time sst upgrade is run it will install the latest sst to the new location which is not on the $PATH and print to the console "upgraded to version XXX" even though running sst version will still pick up the old one in the ~/.sst/bin dir.

Since it's early in the life of sst/ion I wasn't sure if you consider it important to handle this worst case situation more gracefully and want to ask your thoughts before adding extra complexity. If you're ok with just recommending a "clean install" for that situation then I think this PR is ready to merge.

If you think the worst case upgrade scenario needs a better solution, I could do something easy in the upgrade command like

• delete ~/.sst dir if it exists
• check that install bin dir is on the $PATH and if it isn't print a message to the console

Thoughts?