make: immediately expand deferred vars

[Kimplul]

When IN_NIX_SHELL is defined, flow/scripts/variables.mk defines some commonly used variables like OPENROAD_EXE with OPENROAD_EXE ?= $(shell ...). The conditional assignment (?=) is a deferred assignment, so every time OPENROAD_EXE is expanded, the corresponding shell command is run. Apparently some of these variables are expanded often enough that the deferred assignment has a somewhat significant impact on runtime, here's how long make takes to run after having synthesized the Ibex example from the docs:

Before:

$ time IN_NIX_SHELL=1 make DESIGN_CONFIG=./designs/sky130hd/ibex/config.mk
[...]

real    0m52.177s
user    0m42.582s
sys     0m14.881s

The fix here is to immediately re-expand the newly defined variables so the shell command is only ran once.

After:

$ time IN_NIX_SHELL=1 make DESIGN_CONFIG=./designs/sky130hd/ibex/config.mk
[...]

real    0m0.644s
user    0m0.520s
sys     0m0.146s

[rovinski]

I think your methodology is faulty because it's not possible ibex finished in under 1 second. Did you make clean between your measurements?

[Kimplul]

@rovinski To clarify, I'm not running the full Ibex flow, I've previously finished the flow and running make again just prints out resource usages of each stage. I guess a more clear example would be make DESIGN_CONFIG=... elapsed? This PR just removes some unnecessary overhead from running the Makefile.

[maliberty]

There is something pathologically bad about NIX if doing essentially no work takes 50s. @hzeller have observed this?