Use lowercase variable names in bin/build.sh

To prevent naming-collisions, uppercase variables are conventionally
reserved for environment and shell-provided variables.
Modify check-home-bin for convention

* Use [[ built-in consistently
* Move component to just before the task that needs it in Manifest
Avoid sourcing zshrc, manually fix environment

Remove `echo "\n"` usage which is not consistent across shells.
Whitespace and style changes

* Use `if condition; then` for consistency
* Prefer curl over wget for consistency
* Break long lines
Remove zsh-ism, fix logic error

Use more portable `which` command. The condition was also backwards.
Use bash shebang
Remove linux-prerequisite, use bash in test/runner
Set zsh as shell in both scripts
Remove deprecated use of backtick and expr
Update README
Build compiled scripts