Floss: Add TOPSHIM_SHOULD_REBUILD optional env var

Cargo does not have a convenient way to rebuild by specifying wildcard
(e.g. *.h files). In development, topshim will most likely be always
rebuilt since developers likely change any file inside system/. This
adds a development option in environment variable to ignore topshim
rebuild to speed up edit compile cycle when topshim isn't expected to
change.

Bug: 217273154
Tag: #floss
Test: Manual - Build Floss on Linux

Change-Id: Ica667d0d3fdd0229fc7ae8f8a28486e85e09a410