Fix compiler existence checking in case if PATH contains tilde

[kubrack]

Fix (expand_path) compiler existence checking in case if PATH contains tilde:

inav.git/build]$ make MATEKF411
Generating MATEKF411/settings_generated.h, MATEKF411/settings_generated.c
/Users/bojack/tmp/inav.git/src/utils/compiler.rb:60:in `initialize': Could not find arm-none-eabi-g++ in PATH, looked in ["", "~/homebrew/bin", "~/homebrew/sbin", "/opt/local/bin", "/opt/local/sbin", "/usr/local/opt/asdf/libexec/bin", "/usr/local/bin", "/usr/bin", "/bin", "/usr/sbin", "/sbin", "/Library/Apple/usr/bin", "/Users/bojack/bin", "/opt/local/bin", "/opt/local/sbin"] (RuntimeError)

$ ruby -e 'print(File.executable?("~/homebrew/bin/arm-none-eabi-g++"))'
false
$ ruby -e 'print(File.executable?("/Users/bojack/homebrew/bin/arm-none-eabi-g++"))'
true