> I can replicate what you did and it works.

Nice! :-)

> May it be conflicting with other packages?

arm-none-eabi-toolchain is conflicting with avr-toolchain since both are
setting the CROSS_* environment variables. See the output of `avr-gcc -E
-v -` below.

--8<---------------cut here---------------start------------->8---
Using built-in specs.
Reading specs from /gnu/store/cwsiyrcm1c4c25dw0zrgajjnr9mf5k3g-avr-gcc-5.5.0/lib/gcc/avr/5.5.0/device-specs/specs-avr2
COLLECT_GCC=avr-gcc
Target: avr
Configured with: 
Thread model: single
gcc version 5.5.0 (GCC) 
COLLECT_GCC_OPTIONS='-E' '-v' '-specs=device-specs/specs-avr2'
 /gnu/store/cwsiyrcm1c4c25dw0zrgajjnr9mf5k3g-avr-gcc-5.5.0/libexec/gcc/avr/5.5.0/cc1 -E -quiet -v - -mn-flash=6 -mskip-bug
ignoring nonexistent directory "/gnu/store/cwsiyrcm1c4c25dw0zrgajjnr9mf5k3g-avr-gcc-5.5.0/lib/gcc/avr/5.5.0/../../../../avr/sys-include"
ignoring nonexistent directory "/gnu/store/cwsiyrcm1c4c25dw0zrgajjnr9mf5k3g-avr-gcc-5.5.0/lib/gcc/avr/5.5.0/../../../../avr/include"
#include "..." search starts here:
#include <...> search starts here:
 /gnu/store/1bcgk0mwxwsgjpaybvb8h4dd1xp3x19x-profile/arm-none-eabi/include
 /gnu/store/1bcgk0mwxwsgjpaybvb8h4dd1xp3x19x-profile/avr/include
 /gnu/store/cwsiyrcm1c4c25dw0zrgajjnr9mf5k3g-avr-gcc-5.5.0/lib/gcc/avr/5.5.0/include
 /gnu/store/cwsiyrcm1c4c25dw0zrgajjnr9mf5k3g-avr-gcc-5.5.0/lib/gcc/avr/5.5.0/include-fixed
End of search list.
--8<---------------cut here---------------end--------------->8---

This should probably be considered a bug. But I have no idea how to fix
this. :-( What do you think?

Also, our avr-gcc package should probably be updated to the latest
avr-gcc-9.