[nix] add 'dev-mode' attribute to handle script

[Avimitin]

It is incovenient for users to always input strings like ".#t1.blastoise.ip.cases.mlir.hello.emu-result". So here is an env to help improve the current situation. With the "dev-mode" shell, users will have many bash function to help them set the current development "mode".

For example, if users want to test "intrinsic.linear_optimisation" with VCS emulator, and compare the result with verilator-emulator,
they will have to write "nix build
.#t1.blastoise.ip.vcs-emu.cases.intrinsic.linear_optimisation", then ".#t1.blastoise.ip.verilator-emu.cases.intrinsic.linear_optimisation"

With the new script, the test flow is separate into different phrases:

• Firstly, get into the dev-mode shell: nix develop .#dev-mode
• Then, set the config: set_config blastoise
• Users can inspect RTL with simple command too: build vcs-emu-rtl
• To switch between verilator and VCS, they can use: set_emulator vcs-emu or set_emulator verilator-emu
• Finally, run the case: test intrinsic.linear_optimisation

[Avimitin]

Since there are bunch of changes of nix build system in rocket-t1 branch, I would love to hold the PR until the PR #717 is merged, which avoids the need to have 2 PRs for the same functionality.

[Avimitin]

I would love to add this feature into #717 directly.