Fix "expr: syntax error" on macOS

The version of expr shipped on macOS does not support `substr`
command and prints `expr: syntax error`. Change the invocation
of `expr` to use the `:` operator which match rhs regexp into
lhs string and return the length of the match.

This should be supported by all versions of `expr` that are
POSIX compliant.

Bug: 1199260
Change-Id: Ib932702fedbbbc772d65086ebc371dc0a9023c0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2827931
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>

autoninja

@@ -5,7 +5,7 @@
 # found in the LICENSE file.
 
 # In git bash on Windows, invoke the batch file.
-if [ "$(expr substr $(uname -s) 1 10)" == "MINGW64_NT" ]; then
+if [ "$(expr "$(uname -s)" : "^MINGW64_NT")" == "10" ]; then
     autoninja.bat "$@"
     exit
 fi