rcov on macOS 11.0 "big sur" does not install

[pavelz]

error message:

Installing rcov 1.0.0 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/pavel/.rvm/gems/ruby-2.6.5@authzor/gems/rcov-1.0.0/ext/rcovrt
/Users/pavel/.rvm/rubies/ruby-2.6.5/bin/ruby -I /Users/pavel/.rvm/rubies/ruby-2.6.5/lib/ruby/site_ruby/2.6.0 -r ./siteconf20200722-23844-18aa7dw.rb extconf.rb
creating Makefile

current directory: /Users/pavel/.rvm/gems/ruby-2.6.5@authzor/gems/rcov-1.0.0/ext/rcovrt
make "DESTDIR=" clean

current directory: /Users/pavel/.rvm/gems/ruby-2.6.5@authzor/gems/rcov-1.0.0/ext/rcovrt
make "DESTDIR="
compiling 1.8/callsite.c
1.8/callsite.c:2:10: fatal error: 'env.h' file not found
#include <env.h>
         ^~~~~~~
1 error generated.
make: *** [callsite.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/pavel/.rvm/gems/ruby-2.6.5@authzor/gems/rcov-1.0.0 for inspection.
Results logged to /Users/pavel/.rvm/gems/ruby-2.6.5@authzor/extensions/x86_64-darwin-18/2.6.0/rcov-1.0.0/gem_make.out

[ryandesign]

Duplicate of #103.