nix-support: we can build with nix environment

[jensen-yan]

No description provided.

[xieby1]

The newly added nix scripts look promising👍.

HOWEVER, Two problems exist:

1. riscv64-unknown-linux-gnu-gfortran is not presented in shell.nix.
   As a result, several fp test cases cannot compile, including 410, 416, 434, 435, 437, 454, 459,
2. spec2006/bin/specperl has not been patched, therefore cannot run in pure NixOS, non-NixOS probably will not encounter this problem.
   As a result, case 436 failed to compile.

[xieby1]

Fixed riscv64-unknown-linux-gnu-gfortran in eb27204
Fixed spec2006/bin/specperl in 3347ac3