Bump actions/checkout from 5 to 6 (#149) #670

Job	Run time
linux - gfortran-11 - valgrind - push	3m 50s
linux - gfortran-11 - regular - push	24s
linux - gfortran-10 - valgrind - push	3m 31s
linux - gfortran-9 - valgrind - push	3m 40s
linux - gfortran-9 - regular - push	26s
linux - gfortran-12 - regular - push	24s
linux - gfortran-10 - regular - push	56s
linux - gfortran-12 - valgrind - push	3m 36s
macos - gfortran-14 - regular - push	32s
macos - gfortran-15 - regular - push	33s
linux - gfortran-13 - regular - push	23s
macos - gfortran-13 - regular - push	31s
linux - gfortran-14 - valgrind - push	3m 39s
windows - gfortran - regular - push	2m 3s
linux - ifort - regular - push	43s
linux - gfortran-14 - regular - push	22s
linux - gfortran-13 - valgrind - push	4m 1s
	29m 34s

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