fix #12293 findNimStdLibCompileTime should not break with nimble install compiler

[timotheecour]

• fix compiler as a library: findNimStdLibCompileTime fails to find system.nim #12293
• there are still some subtleties as mentioned in compiler as a library: findNimStdLibCompileTime fails to find system.nim #12293 (comment)
  but this should work at least:

# with nimble develop
cd $nim
NIMBLE_DIR=$HOME/.nimble_fake24 nimble develop
# or, with nimble install for any version recent enough to have this patch:
NIMBLE_DIR=$HOME/.nimble_fake24 nimble install https://github.com/timotheecour/Nim@#pr_fix_findNimStdLibCompileTime

# then:
NIMBLE_DIR=$HOME/.nimble_fake24 nim r $timn_D/tests/nim/all/t10738.nim

# t10738.nim:
import compiler/[nimeval, llstream]
import os
proc evalString(code: string, moduleName = "script.nim") =
  let stream = llStreamOpen(code)
  let lib = findNimStdLibCompileTime()
  var intr = createInterpreter(moduleName, [lib, lib / "pure", lib / "core"])
  intr.evalScript(stream)
  destroyInterpreter(intr)
  llStreamClose(stream)

let runtimeString = "echo \"Hello World\""
evalString(runtimeString)

[timotheecour]

PTAL
CI failure unrelated: #14090 re-ran