env/darwin: delete binary before redownloading it

When a new buildlet version becomes available and gets downloaded,
we don't want the previous file attributes to persist, since that
can cause the new binary to get killed at startup. Delete the old
file before downloading a new one to avoid such problems.

Also sleep a bit longer so output is easier to read.

Change-Id: Id4d95e42c033e82377b9eef636fa6a154af6cb54
Reviewed-on: https://go-review.googlesource.com/c/build/+/419980
Auto-Submit: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dmitshur

env/darwin/setup-notes.md

@@ -50,12 +50,13 @@ set -x
 
 mkdir -p ~/go/bin;
 while true; do
+  rm -f ~/go/bin/buildlet
   url="https://storage.googleapis.com/go-builder-data/buildlet.darwin-arm64"
   while ! curl -f -o ~/go/bin/buildlet "$url"; do
       echo
       echo "curl failed to fetch $url"
       echo "Sleeping before retrying..."
-      sleep 2
+      sleep 5
   done
   chmod +x ~/go/bin/buildlet