x/build: NetBSD builder needs a bigger /tmp partition

[bcmills]

The bootstrap test added in https://golang.org/cl/167478 is causing the netbsd-amd64-8_0 builder to fail:

##### ../misc/reboot
Building Go cmd/dist using /tmp/workdir/go.
Building Go toolchain1 using /tmp/workdir/go.
Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
Building Go toolchain2 using go_bootstrap and Go toolchain1.
# cmd/cgo
/tmp/workdir/tmp/reboot-goroot029710423/pkg/tool/netbsd_amd64/link: flushing $WORK/b059/exe/a.out: write $WORK/b059/exe/a.out: no space left on device
# cmd/link/internal/loadxcoff
compile: writing output: write $WORK/b126/_pkg_.a: no space left on device
# go/doc
compile: writing output: write $WORK/b101/_pkg_.a: no space left on device
# cmd/link/internal/objfile
compile: writing output: write $WORK/b127/_pkg_.a: no space left on device
# cmd/compile/internal/ssa
compile: writing output: write $WORK/b084/_pkg_.a: no space left on device
go tool dist: FAILED: /tmp/workdir/tmp/reboot-goroot029710423/pkg/tool/netbsd_amd64/go_bootstrap install -gcflags=all= -ldflags=all= -i cmd/asm cmd/cgo cmd/compile cmd/link: exit status 2
--- FAIL: TestRepeatBootstrap (69.75s)
    reboot_test.go:50: exit status 2
FAIL

It looks like it just needs more space in /tmp to write the bootstrap binaries.

CC @dmitshur @bradfitz @bsiegert

[bsiegert]

/tmp on this builder is on the root file system, so we need a bigger root fs.

[gopherbot]

Change https://golang.org/cl/167637 mentions this issue: dashboard: skip the "reboot" dist test on NetBSD due to disk space issues