Refine error mesasage "attempt to index a nil value" about l3build HOT 4 CLOSED

@koppor Ah, MiKTeX :) @u-fischer mentioned that we should look at this again (basically, it doesn't work as MiKTeX doesn't return the same info as TeX Live). I promised I'd look at her issue on my way back, so I'll add this to the immediate list.

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I've reported to MiKTeX: I think we can't really do anything here as it's not an l3build bug.

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It's not out fault :) You can minimise to

--[[

% !TeX program = texlua

--]]

-- io.popen("echo","w"):write(""):close() -- Works
-- io.popen("echo","w"):write("")         -- Works
io.popen("echo","w"):write("y"):close()

which as I've commented works if you don't have any content to the write or don't close the stream, but if you have both content and closing. Do you want to report against MiKTeX or should I?

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Note that if you bypass the issue in a locally-edited l3build, running the tests for l3build itself

texlua .\l3build.lua check -epdftex -cbuild

shows two other problems. First, for some reason not all of the tests are picked up. Second, the allowed variation if glue setting pops up. Thus, TeX Live on Windows gives

  00-test-1 (1/2)
  01-expect-1 (2/2)

  All checks passed

but MiKTeX gives

Running checks on
  00-test-1 (1/1)
          --> failed

An example line that changes (TeX Live first):

...\hbox(6.94444+0.83333)x469.75499, glue set 427.75536fil
...\hbox(6.94444+0.83333)x469.75499, glue set 427.75537fil

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