bootstrap.bash: make source writable before cleaning

Otherwise the "git clean" command fails with errors like
    rm: cannot remove '/home/iant/go-linux-ppc64-bootstrap/pkg/mod/golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c/encoding/simplifiedchinese/all.go': Permission denied

Change-Id: Iecfb1fed6d59819d7fdceb9e391a2b3f81ea620c
Reviewed-on: https://go-review.googlesource.com/c/go/+/172998
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bootstrap.bash

@@ -49,6 +49,7 @@ cp -R "$src" "$targ"
 cd "$targ"
 echo
 echo "#### Cleaning $targ"
+chmod -R +w .
 rm -f .gitignore
 if [ -e .git ]; then
 	git clean -f -d