Use root account for soroban upgrades in loadgen #6364

Job	Run time
rust-check-git-rev-deps	18s
cargo-deny (advisories)	50s
build (gcc, current)	38m 29s
fmt	16s
cargo-deny (bans licenses sources)	45s
build (gcc, current, --check-test-tx-meta)	26m 6s
build (gcc, next)	36m 49s
build (gcc, next, --check-test-tx-meta)	24m 23s
build (clang, current)	29m 54s
build (clang, current, --check-test-tx-meta)	19m 36s
build (clang, next)	28m 20s
build (clang, next, --check-test-tx-meta)	20m 49s
complete	1s
	3h 46m 36s

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