Make it possible to choose using host-provided capnproto #662

Job	Run time
Linux \| Coverage \| gcc \| debug	1s
MSYS2 (MSYS) \| Install \| gcc \| release	1s
Linux \| Install \| gcc \| debug	1s
Linux \| Install \| gcc \| release	1s
Linux \| Install \| clang \| debug	1s
Linux \| Install \| clang \| release	1s
CodeFormatting	1s
ClangTidy	1s
Linux \| Python API \| gcc \| release	1s
Mac OS \| Install \| gcc \| release	1s
Mac OS \| Install \| clang \| release	1s
Version Tagging	0s
Windows \| Install \| cl \| release	1s
Windows \| Install \| clang \| release	1s
Linux \| Regression \| ${{ matrix.compiler }} \| ${{ matrix.config }} [${{ matrix.shard }}]	1s
Linux \| Valgrind \| ${{ matrix.project }} \| ${{ matrix.compiler }} \| ${{ matrix.config }}	1s
MSYS2 (MSYS) \| Regression \| ${{ matrix.compiler }} \| ${{ matrix.config }} [${{ matrix.shard }}]	1s
Mac OS \| Regression \| ${{ matrix.compiler }} \| ${{ matrix.config }} [${{ matrix.shard }}]	1s
Windows \| Regression \| ${{ matrix.compiler }} \| ${{ matrix.config }} [${{ matrix.shard }}]	1s
	18s

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