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[LinuxVersion] Use "Construct on first use" idiom and add mutex

Use the "Construct on first use" idom for UTS Release (aka Linux version) initialization. The previous implementation was fragle and could lead to segfaults like

__s2=0x7fffffffe480 "5.16.10-arch1-1", __s1=<optimized out>)
at /usr/include/c++/11.2.0/bits/char_traits.h:409
__n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
at /usr/include/c++/11.2.0/bits/basic_string.h:359
__n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
at /usr/include/c++/11.2.0/bits/basic_string.h:354
__str=..., this=0x7ffff7fbd2a0 <emper::lib::LinuxVersion::globalVersion[abi:cxx11]>)
at /usr/include/c++/11.2.0/bits/basic_string.h:739
at ../emper/lib/LinuxVersion.cpp:46
at /usr/include/c++/11.2.0/ext/new_allocator.h:79

if globalVersion's constructor was not yet called. This is the so called "static initialization order fiasco" in C++.

While the mutex may not be strictly requires, as we do not call LinuxVersion's non-argument constructor nor getUtsRelease() concurrently, it can not hurt to have one.

Closes !347 (closed).

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