internal/driver: avoid using $HOME directly

Not all OSes have $HOME variable.
Use appropriate variables per OS.

Fixes https://github.com/golang/go/issues/18864

doc/pprof.md

 
 * **-text:** Prints the location entries, one per line, including the flat and cum
   values.
-* **-tree:** Prints each location entry with its predecessors and successors. 
+* **-tree:** Prints each location entry with its predecessors and successors.
 * **-peek= _regex_:** Print the location entry with all its predecessors and
   successors, without trimming any entries.
 * **-traces:** Prints each sample with a location per line.
 
 pprof will look for source files on its current working directory and all its
 ancestors. pprof will look for binaries on the directories specified in the
-`$PPROF_BINARY_PATH` environment variable, by default `$HOME/pprof/binaries`. It
-will look binaries up by name, and if the profile includes linker build ids, it
-will also search for them in a directory named as the build id.
+`$PPROF_BINARY_PATH` environment variable, by default `$HOME/pprof/binaries`
+(`%USERPROFILE%\pprof\binaries` on Windows). It will look binaries up by name,
+and if the profile includes linker build ids, it will also search for them in
+a directory named as the build id.
 
 pprof uses the binutils tools to examine and disassemble the binaries. By
 default it will search for those tools in the current path, but it can also

internal/driver/cli.go

 	"   PPROF_TOOLS        Search path for object-level tools\n" +
 	"   PPROF_BINARY_PATH  Search path for local binary files\n" +
 	"                      default: $HOME/pprof/binaries\n" +
-	"                      finds binaries by $name and $buildid/$name\n"
+	"                      finds binaries by $name and $buildid/$name\n" +
+	"   * On Windows, %USERPROFILE% is used instead of $HOME"

internal/driver/fetch.go

 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
 	err    error
 }
 
+func homeEnv() string {
+	switch runtime.GOOS {
+	case "windows":
+		return "USERPROFILE"
+	case "plan9":
+		return "home"
+	default:
+		return "HOME"
+	}
+}
+
 // setTmpDir prepares the directory to use to save profiles retrieved
 // remotely. It is selected from PPROF_TMPDIR, defaults to $HOME/pprof.
 func setTmpDir(ui plugin.UI) (string, error) {
 	if profileDir := os.Getenv("PPROF_TMPDIR"); profileDir != "" {
 		return profileDir, nil
 	}
-	for _, tmpDir := range []string{os.Getenv("HOME") + "/pprof", os.TempDir()} {
+	for _, tmpDir := range []string{os.Getenv(homeEnv()) + "/pprof", os.TempDir()} {
 		if err := os.MkdirAll(tmpDir, 0755); err != nil {
 			ui.PrintErr("Could not use temp dir ", tmpDir, ": ", err.Error())
 			continue
 	searchPath := os.Getenv("PPROF_BINARY_PATH")
 	if searchPath == "" {
 		// Use $HOME/pprof/binaries as default directory for local symbolization binaries
-		searchPath = filepath.Join(os.Getenv("HOME"), "pprof", "binaries")
+		searchPath = filepath.Join(os.Getenv(homeEnv()), "pprof", "binaries")
 	}
 mapping:
 	for _, m := range p.Mapping {