nix-shell: don't use XDG_RUNTIME_DIR if TMPDIR is unset #2728
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XDG_RUNTIME_DIR isn't meant for possibly-large build processes; it's
mostly meant for small sockets, named pipes, and other IPC
mechanisms. As a result it's limited in size on many systems, and so
using it as a temporary directory (by setting TMPDIR to it) will cause
some test and build processes to fail when they try to use more space
than is available in XDG_RUNTIME_DIR.
For example, XDG_RUNTIME_DIR is limited to 100MB on an older Debian
system, and some build and test processes try to use several GB of
space for temporary artifacts; these failures are spurious, and can be
avoided if we just default directly to /tmp instead of trying to
default to XDG_RUNTIME_DIR first.