This option controls the permissions of installed files (except for
those specified explicitly using install_mode option, e.g. in
install_data rules.)
An install-umask of 022 will install all binaries, directories and
executable files with mode rwxr-xr-x, while all data and non-executable
files will be installed with mode rw-r--r--.
Setting install-umask to the string 'preserve' will disable this
feature, keeping the permissions of installed files same as the files in
the build tree (or source tree for install_data and install_subdir.)
Note that, in this case, the umask used when building and that used when
checking out the source tree will leak into the install tree.
Keep the default as 'preserve', to show that no behavior is changed and
all tests keep passing unchanged.
Tested: ./run_tests.py
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