Restore previous behavior of `get_build_var TMPDIR`

My previous change makes TMPDIR a temporary directory that's removed
before we exit. So instead of starting ckati to get this value to a
directory that no longer exists, return the semi-stable value that we
use during a normal build.

Fixes: 142277335
Test: get_build_var TMPDIR
Change-Id: I37d1219b1fda09f131dee17ef7b91e475d3e39ad

ui/build/dumpvars.go

@@ -42,6 +42,7 @@ func DumpMakeVars(ctx Context, config Config, goals, vars []string) (map[string]
 	soongUiVars := map[string]func() string{
 		"OUT_DIR":  func() string { return config.OutDir() },
 		"DIST_DIR": func() string { return config.DistDir() },
+		"TMPDIR":   func() string { return absPath(ctx, config.TempDir()) },
 	}
 
 	makeVars := make([]string, 0, len(vars))