Use main branch for flutter engine clones

The master branch was renamed to main.

Change-Id: Ib38f3976b497c17e0bda904cb99864e127016272
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/311153
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Auto-Submit: Nate Bosch <nbosch@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>

Author: natebosch

tools/bots/flutter/analyze_flutter_engine.sh

@@ -17,7 +17,7 @@ cleanup() {
 trap cleanup EXIT HUP INT QUIT TERM PIPE
 cd "$tmpdir"
 
-git clone --single-branch -vv \
+git clone --single-branch --branch main -vv \
   https://dart.googlesource.com/external/github.com/flutter/engine
 
 cd engine

tools/bots/flutter/compile_flutter.sh

@@ -71,7 +71,7 @@ popd  # flutter
 git clone --single-branch --depth=1 -vv \
     https://dart.googlesource.com/external/github.com/flutter/buildroot src
 pushd src
-git clone --single-branch --depth=1 -vv \
+git clone --single-branch --branch main --depth=1 -vv \
     https://dart.googlesource.com/external/github.com/flutter/engine flutter
 mkdir -p third_party
 pushd third_party