Update `build.sh` to error-out on bad `$BUILD` or `$BIN`

[wilsonjholmes]

I tried to install to a directory that did not exist, and I got some unexpected behavior...

build.sh did not fail, it just continued to run and spew-out errors.

I added a -e after the bash sh-bang to help with catching issues sooner (portion of man page pasted below), and I changed readlink -f to realpath --canonicalize-existing (man page pasted also):

-e Exit immediately if a pipeline (which may consist of a single simple command), a list, or a com‐ pound command (see SHELL GRAMMAR above), exits with a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test following the if or elif reserved words, part of any command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command's return value is being inverted with !. If a compound command other than a sub‐ shell returns a non-zero status because a command failed while -e was being ignored, the shell does not exit. A trap on ERR, if set, is executed before the shell exits. This option applies to the shell environment and each subshell environment separately (see COMMAND EXECUTION ENVIRONMENT above), and may cause subshells to exit before executing all the commands in the subshell.

READLINK(1) User Commands READLINK(1)

NAME readlink - print resolved symbolic links or canonical file names

SYNOPSIS readlink [OPTION]... FILE...

DESCRIPTION Note realpath(1) is the preferred command to use for canonicalization functionality.

Honestly, not sure if this would work on all POSIX-like systems that the project is targeting, but it built for me on Pop!_OS 22.04 LTS and klayout launches.

I am open to suggestions or learnings if realpath or -e is not suitable for some reason.