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fix: COPY .cargo/ into both image builders (regression from #313), and correct the RUSTFLAGS fallback tier #886

Job	Run time
hpc-stream-parallel/rayon	1m 25s
clippy/1.98.1	38s
nostd/thumbv6m-none-eabi/stable	32s
format/stable	24s
wasm-simd/parity-node	1m 1s
tier4-avx512-check	53s
native-backend/stable	6m 54s
Pass MSRV values to other jobs	3s
neon-simd/parity-qemu	1m 39s
miri	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
nightly-simd-polyfill	-1s
docs/nightly	-1s
cargo-careful	-1s
blas-msrv	48s
tests/stable	10m 50s
tests/beta	11m 13s
tests/1.98.1	10m 44s
conclusion	0s
	47m 1s