Fix filepath formatting for Windows in update-microkernels.py #252

Job	Run time
build-and-test / bazel-linux-x86_64-gcc-9	5m 2s
build-and-test / bazel-linux-aarch64	4m 38s
build-and-test / bazel-linux-x86_64-clang-18	6m 36s
build-and-test / cmake-android (arm64)	1m 53s
build-and-test / cmake-android (armv7)	2m 12s
build-and-test / cmake-ios-x86_64	6m 12s
build-and-test / cmake-ios-arm64	7m 48s
build-and-test / cmake-android (x86)	2m 4s
build-and-test / cmake-linux-aarch64	3m 38s
build-and-test / cmake-linux-armhf	2m 53s
build-and-test / cmake-linux-riscv64	7m 47s
build-and-test / cmake-linux-riscv64-rvv	10m 0s
build-and-test / cmake-linux-x86_64	2m 49s
build-and-test / cmake-macos-arm64	1m 20s
build-and-test / cmake-windows-arm64	2m 59s
build-and-test / cmake-windows-x64	3m 28s
build-and-test / cmake-windows-x86	4m 23s
	1h 15m 42s

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