fetchcvs: fix ssh wrapper failing due to missing /usr/bin/env

/usr/bin/env seems to be no longer be present in the sandbox. This means
that fetchcvs would fail with a “not found error” whenever CVS_RSH was
necessary.

We fix this by simply setting the current $SHELL as shebang.
Alternatively also setting it to /bin/sh statically would be possible.

pkgs/build-support/fetchcvs/builder.sh

@@ -1,6 +1,6 @@
 source $stdenv/setup
 
-(echo '#!/usr/bin/env sh'; \
+(echo "#!$SHELL"; \
  echo 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no "$@"') > ssh
 chmod +x ssh
 export CVS_RSH=$PWD/ssh