emit_size() rule in rules/shell_command/rules.bzl fails on macOS

[laszlocsomor]

https://buildkite.com/bazel/bazel-bazel-examples/builds/54#d3ab95d6-7c36-473b-a4ea-935ccd30e105

(02:04:43) ERROR: /Users/buildkite/builds/bk-imacpro-13/bazel/bazel-bazel-examples/rules/shell_command/BUILD:10:1: Couldn't build file shell_command/foo.size: error executing shell command: '/bin/bash -c stat -L -c%s 'shell_command/foo.txt' > 'bazel-out/darwin-fastbuild/bin/shell_command/foo.size'' failed (Exit 1) bash failed: error executing command
--
  | (cd /private/var/tmp/_bazel_buildkite/498b38e3f668fa53ddf838edc27b273f/sandbox/darwin-sandbox/7/execroot/__main__ && \
  | exec env - \
  | /bin/bash -c 'stat -L -c%s '\''shell_command/foo.txt'\'' > '\''bazel-out/darwin-fastbuild/bin/shell_command/foo.size'\''')
  | Execution platform: @bazel_tools//platforms:host_platform
  |  
  | Use --sandbox_debug to see verbose messages from the sandbox
  | stat: illegal option -- c
  | usage: stat [-FlLnqrsx] [-f format] [-t timefmt] [file ...]

The rule executes stat -L -c%s <path> but -c is not supported on macOS.

[laszlocsomor]

stat -L -f%z <path> is the right incantation