www/firefox-71.0 configuration failure on FreeBSD

[Frédéric Fauberteau <triaxx%NetBSD.org@localhost>]

Now I have graphics/MesaLib and lang/rust building on FreeBSD, I tried 
to build www/firefox.

First, I try to build using pbulk in a sandbox but I get an error during 
configuration phase:
=> Generating pkg-config files for builtin xz package.
env: -E: No such file or directory
*** Error code 127

The complete log: 
http://pkg.triaxx.org/pub/pkgsrc/reports/FreeBSD/x86_64/12.0/test/20200102.2147/firefox-71.0/configure.log

In order to identify the problem, I tried to secondly build out of 
chroot environment. But for an unknown reason, I got another error:
mozbuild.frontend.reader.SandboxValidationError:
==============================
FATAL ERROR PROCESSING MOZBUILD FILE
==============================

The error occurred while processing the following file or one of the 
files it includes:

    
/usr/pkgsrc/www/firefox/work/firefox-71.0/media/libcubeb/src/moz.build

The error occurred when validating the result of the execution. The 
reported error is:

    File listed in SOURCES does not exist: 
'/usr/pkgsrc/www/firefox/work/firefox-71.0/media/libcubeb/src/cubeb_oss.c'

The complete 
log:http://pkg.triaxx.org/pub/pkgsrc/logs/devel/firefox/firefox-71.0/configure.log

1) The difference of behavior between pbulk/sandbox environment and 
classical environment is really surprising

2) $ find work/firefox-71.0/ -name "cubeb_oss.c" gives me nothing