CI: Use python3 explicitly in pr-closed workflow #881

Job	Run time
build_gcc_arm / build	1s
checks / checks	23s
build_llvm_x86_64 / build	1s
build_gcc_x86_64 / build	1s
build_gcc_aarch64 / build	1s
assert_checks	1s
compile_devicetrees	0s
build_gcc_arm_zynq_m2k_defconfig	0s
deploy_cloudsmith_checks	0s
build_gcc_arm_socfpga_adi_defconfig	0s
build_gcc_aarch64_adi_zynqmp_defconfig	0s
build_gcc_aarch64_adi_versal_defconfig	0s
build_gcc_arm_zynq_pluto_defconfig	0s
build_gcc_arm_zynq_xcomm_adv7511_defconfig	0s
build_gcc_nios2_adi_nios2_defconfig	0s
build_gcc_microblaze_adi_mb_defconfig	0s
assert_build	1s
deploy_cloudsmith_build	0s
	29s

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