feat: nix devshell

[machshev]

Trial of a new EDA devshell mechanism introduced in lowRISC/lowrisc-nix#189

When $LOWRISC_EDA_CONFIG is set to point to the lowrisc internal EDA config I was able to run the earlgrey smoke tests with dvsim.

> nix develop .
> dvsim hw/top_earlgrey/dv/top_earlgrey_sim_cfgs.hjson -i smoke --fixed-seed 1 --cov
...
          [   legend    ]: [S: scheduled, Q: queued, R: running, P: passed, F: failed, K: killed, T: total]
00:11:43  [    build    ]: [S:       000, Q:    000, R:     000, P:    072, F:    017, K:    000, T:   089] 100%
00:11:43  [     run     ]: [S:       000, Q:    000, R:     000, P:    106, F:    009, K:    039, T:   154] 100%
00:11:43  [  cov_merge  ]: [S:       000, Q:    000, R:     000, P:    036, F:    000, K:    011, T:   047] 100%
00:11:43  [ cov_report  ]: [S:       000, Q:    000, R:     000, P:    036, F:    000, K:    011, T:   047] 100%
00:09:33  [  cov_vplan  ]: [S:       000, Q:    000, R:     000, P:    001, F:    000, K:    000, T:   001] 100%

[rswarbrick]

This looks very sensible to me (and has zero risk to the rest of the project, so I can't really see any downside to wiring it up).