scram-project-build: fix quoting for USER_CXXFLAGS

The double quoting used for USER_CXXFLAGS passed multiple parameters to
GCC as a single one. Remove not needed quotes.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Tested-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>