[deb] Avoid UTF-8 roundtrip for {pre,post}-{inst,rm} hooks

[Yannic]

This change allows the Debian package hooks to be arbitrary files (including executable binary files), which failed with the following error before:

Traceback (most recent call last):
  File "/Users/yannic/Library/Caches/bazel/_bazel_yannic/bb6c3b02ccc98aaca966df445b5024eb/sandbox/darwin-sandbox/3472/execroot/_main/bazel-out/darwin_x86_64-opt-exec/bin/external/rules_pkg+/pkg/private/deb/make_deb.runfiles/rules_pkg+/pkg/private/deb/_make_deb_stage2_bootstrap.py", line 499, in <module>
    main()
    ~~~~^^
  File "/Users/yannic/Library/Caches/bazel/_bazel_yannic/bb6c3b02ccc98aaca966df445b5024eb/sandbox/darwin-sandbox/3472/execroot/_main/bazel-out/darwin_x86_64-opt-exec/bin/external/rules_pkg+/pkg/private/deb/make_deb.runfiles/rules_pkg+/pkg/private/deb/_make_deb_stage2_bootstrap.py", line 493, in main
    _run_py_path(main_filename, args=sys.argv[1:])
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/yannic/Library/Caches/bazel/_bazel_yannic/bb6c3b02ccc98aaca966df445b5024eb/sandbox/darwin-sandbox/3472/execroot/_main/bazel-out/darwin_x86_64-opt-exec/bin/external/rules_pkg+/pkg/private/deb/make_deb.runfiles/rules_pkg+/pkg/private/deb/_make_deb_stage2_bootstrap.py", line 287, in _run_py_path
    runpy.run_path(main_filename, run_name="__main__")
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen runpy>", line 287, in run_path
  File "<frozen runpy>", line 98, in _run_module_code
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/yannic/Library/Caches/bazel/_bazel_yannic/bb6c3b02ccc98aaca966df445b5024eb/sandbox/darwin-sandbox/3472/execroot/_main/bazel-out/darwin_x86_64-opt-exec/bin/external/rules_pkg+/pkg/private/deb/make_deb.runfiles/rules_pkg+/pkg/private/deb/make_deb.py", line 433, in <module>
    main()
    ~~~~^^
  File "/Users/yannic/Library/Caches/bazel/_bazel_yannic/bb6c3b02ccc98aaca966df445b5024eb/sandbox/darwin-sandbox/3472/execroot/_main/bazel-out/darwin_x86_64-opt-exec/bin/external/rules_pkg+/pkg/private/deb/make_deb.runfiles/rules_pkg+/pkg/private/deb/make_deb.py", line 392, in main
    prerm=helpers.GetFlagValue(options.prerm, False),
          ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/yannic/Library/Caches/bazel/_bazel_yannic/bb6c3b02ccc98aaca966df445b5024eb/sandbox/darwin-sandbox/3472/execroot/_main/bazel-out/darwin_x86_64-opt-exec/bin/external/rules_pkg+/pkg/private/deb/make_deb.runfiles/rules_pkg+/pkg/private/helpers.py", line 81, in GetFlagValue
    flagvalue = f.read().decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xcf in position 0: invalid continuation byte

(the example used a go_binary for prerm)

[cgrindel]

Looks reasonable to me. I will leave it open for @aiuto to review.