portable-formulae: reset HOMEBREW_DYNAMIC_LINKER on Linux
portable-formulae: reset HOMEBREW_DYNAMIC_LINKER on Linux

This prevents linuxbrew's superenv passing
`-Wl,--dynamic-linker=#{HOMEBREW_PREFIX}/lib/ld.so` to the compiler

Fixes #32.

xu-cheng/homebrew-portable,Homebrew/homebrew-portable