[builder] Meaningfully named temporary files

[simoncozens]

The builder uses a lot of temporary files, and when debugging a problem, it's a pain to trace through the build.ninja file to determine how /tmp/4p0m9zvx2l739 was produced from /tmp/tdpflm38kv4. With this PR, when the builder config logLevel is DEBUG, files are named builder-<target>-<source>-<steps>. e.g. when you see:

/tmp/builder-PlaywriteAUNSW[wght].ttf-Playwrite_MM.glyphspackage-buildVariable-fix-buildStat-subspace

you know you are building PlaywriteAUNSW[wght].ttf and so far you have started with Playwrite_MM.glyphspackage and run the steps buildVariable, fix, buildStat and subspace.