$ guix environment --fallback -s i686-linux --pure mes
[...Building gcc-cross-sans-libc-i686 7.5.0 i686...]
[...]
The directory that should contain system headers does not exist:
  /usr/include
make[3]: *** [Makefile:3009: stmp-fixinc] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory '/tmp/guix-build-gcc-cross-sans-libc-i686-unknown-linux-gnu-7.5.0.drv-0/build/gcc'
make[2]: *** [Makefile:4492: all-stage1-gcc] Error 2
make[2]: Leaving directory '/tmp/guix-build-gcc-cross-sans-libc-i686-unknown-linux-gnu-7.5.0.drv-0/build'
make[1]: *** [Makefile:17230: stage1-bubble] Error 2
make[1]: Leaving directory '/tmp/guix-build-gcc-cross-sans-libc-i686-unknown-linux-gnu-7.5.0.drv-0/build'
make: *** [Makefile:881: all] Error 2
command "make" "-j" "4" "CFLAGS=-g0 -O2" failed with status 2

$ LC_ALL=C guix describe
Generation 96   May 31 2020 22:55:17    (current)
  guix 620daa6
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 620daa6b342d4d72fef2d9ce1ba2fbd11a65266b