Changes

fix(tikv): fix cargo-nextest architecture mismatch in unit-test-coverage job (#4032)

## Issue
The unit-test-coverage postsubmit job of tikv/tikv encountered the
following error during execution: `cannot execute binary file: Exec
format error`.
## Cause
The image supports multiple architectures, but the script hardcodes the
aarch64 cargo-nextest binary, resulting in execution failure on x86_64
nodes.
## Fix
Add dynamic architecture detection to automatically select the
corresponding cargo-nextest binary file based on the actual architecture
of the running node.
 (commit: 4724dd6)