runInLinuxVM: fix for structured attrs

Closes NixOS#334705
Addresses NixOS#205690

The main issue was that the output variable (i.e. `$out` and friends)
didn't exist. I figured the easiest way to add those is to source
`stdenv` here. Given that we build another derivation in this builder,
it's pretty likely that `stdenv` gets pulled already, so I don't expect
a real overhead here.

Also, this mounts `/build` into the VM: this is required to make sure
`.attrs.json` & `.attrs.sh` are available. Dropped the mount of `xchg`
into `/tmp` now since it's also part of `/build`.

pkgs/build-support/vm/default.nix

@@ -130,15 +130,15 @@ rec {
     mkdir -p /fs${storeDir}
     mount -t 9p store /fs${storeDir} -o trans=virtio,version=9p2000.L,cache=loose,msize=131072
 
+    echo "mounting host's build directory..."
+    mkdir -p /fs/build
+    mount -t 9p sa /fs/build -o trans=virtio,version=9p2000.L,cache=loose,msize=131072
+
     mkdir -p /fs/tmp /fs/run /fs/var
     mount -t tmpfs -o "mode=1777" none /fs/tmp
     mount -t tmpfs -o "mode=755" none /fs/run
     ln -sfn /run /fs/var/run
 
-    echo "mounting host's temporary directory..."
-    mkdir -p /fs/tmp/xchg
-    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,msize=131072
-
     mkdir -p /fs/proc
     mount -t proc none /fs/proc
 
@@ -169,7 +169,9 @@ rec {
 
   stage2Init = writeScript "vm-run-stage2" ''
     #! ${bash}/bin/sh
-    source /tmp/xchg/saved-env
+    source /build/xchg/saved-env
+    [ -f "''${NIX_ATTRS_SH_FILE-}" ] && source "$NIX_ATTRS_SH_FILE"
+    source $stdenv/setup
 
     # Set the system time from the hardware clock.  Works around an
     # apparent KVM > 1.5.2 bug.
@@ -203,8 +205,8 @@ rec {
     if test -n "$origBuilder" -a ! -e /.debug; then
       exec < /dev/null
       ${coreutils}/bin/touch /.debug
-      $origBuilder $origArgs
-      echo $? > /tmp/xchg/in-vm-exit
+      $origBuilder ''${origArgs[*]}
+      echo $? > /build/xchg/in-vm-exit
 
       ${busybox}/bin/mount -o remount,ro dummy /
 
@@ -223,6 +225,7 @@ rec {
       -nographic -no-reboot \
       -device virtio-rng-pci \
       -virtfs local,path=${storeDir},security_model=none,mount_tag=store \
+      -virtfs local,path=/build,security_model=none,mount_tag=sa \
       -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \
       ''${diskImage:+-drive file=$diskImage,if=virtio,cache=unsafe,werror=report} \
       -kernel ${kernel}/${img} \
@@ -233,6 +236,8 @@ rec {
 
 
   vmRunCommand = qemuCommand: writeText "vm-run" ''
+    [ -f "''${NIX_ATTRS_SH_FILE-}" ] && source "$NIX_ATTRS_SH_FILE"
+    source $stdenv/setup
     export > saved-env
 
     PATH=${coreutils}/bin