#4770 Do not force sudo when it is not found

[uilianries]

When CONAN_SYSREQUIRES_SUDO is not specified and sudo is not listed by tool.which, SystemPackageTools should not set sudo as prefix.

Changelog: Fix: SystemPackageTools doesn't run sudo when it's not found (#4470)
Docs: conan-io/docs#1127
fixes: #4770

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