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

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs