CI: Set codepage to utf-8 for msys2 tests run. #3791

Job	Run time
cmake-offline-googletest-src	4m 19s
cmake-offline-googletest	4m 17s
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	4m 23s
system-sexpp, sexpp shared libs on, rnp shared libs on	2m 49s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 29s
system-sexpp, sexpp shared libs on, rnp shared libs off	2m 47s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 9s
system-sexpp, sexpp shared libs off, rnp shared libs on	3m 4s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	4m 7s
system-sexpp, sexpp shared libs off, rnp shared libs off	3m 8s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	3m 53s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 16s
package-source	1m 7s
package	1m 2s
debian-tests	13s
	48m 3s

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