Error: fetch /5354.tiflash.json

## Bug Report

### 1. Minimal reproduce step (Required)

```sh
tiup cluster deploy local-tidb v7.6.0 ./topo.yaml --user root -p
Input SSH password:

+ Detect CPU Arch Name
- Detecting node 172.16.8.146 Arch info ... Done

+ Detect CPU OS Name
- Detecting node 172.16.8.146 OS info ... Done
Please confirm your topology:
Cluster type: tidb
Cluster name: local-tidb
Cluster version: v7.6.0
Role Host Ports OS/Arch Directories
---- ---- ----- ------- -----------
pd 172.16.8.146 2379/2380 linux/x86_64 /home/silviu/Desktop/apps/tidb/tidb-deploy/pd-2379,/home/silviu/Desktop/apps/tidb/tidb-data/pd-2379
tikv 172.16.8.146 20160/20180 linux/x86_64 /home/silviu/Desktop/apps/tidb/tidb-deploy/tikv-20160,/home/silviu/Desktop/apps/tidb/tidb-data/tikv-20160
tidb 172.16.8.146 4000/10080 linux/x86_64 /home/silviu/Desktop/apps/tidb/tidb-deploy/tidb-4000
tiflash 172.16.8.146 9000/8123/3930/20170/20292/8234 linux/x86_64 /home/silviu/Desktop/apps/tidb/tidb-deploy/tiflash-9000,/home/silviu/Desktop/apps/tidb/tidb-data/tiflash-9000
prometheus 172.16.8.146 9090/12020 linux/x86_64 /home/silviu/Desktop/apps/tidb/tidb-deploy/prometheus-9090,/home/silviu/Desktop/apps/tidb/tidb-data/prometheus-9090
grafana 172.16.8.146 3000 linux/x86_64 /home/silviu/Desktop/apps/tidb/tidb-deploy/grafana-3000
Attention:
1. If the topology is not what you expected, check your yaml file.
2. Please confirm there is no port/directory conflicts in same host.
Do you want to continue? [y/N]: (default=N) y
+ Generate SSH keys ... Done
+ Download TiDB components
- Download pd:v7.6.0 (linux/amd64) ... Done
- Download tikv:v7.6.0 (linux/amd64) ... Done
- Download tidb:v7.6.0 (linux/amd64) ... Done
- Download tiflash:v7.6.0 (linux/amd64) ... Error
- Download prometheus:v7.6.0 (linux/amd64) ... Done
- Download grafana:v7.6.0 (linux/amd64) ... Done
- Download node_exporter: (linux/amd64) ... Done
- Download blackbox_exporter: (linux/amd64) ... Done

Error: fetch /5354.tiflash.json from mirror(https://tiup-mirrors.pingcap.com) failed: download from https://tiup-mirrors.pingcap.com/5354.tiflash.json failed: unexpected EOF
```

Bootstraping the cluster fails with:
```
Error: fetch /5354.tiflash.json from mirror(https://tiup-mirrors.pingcap.com) failed: download from https://tiup-mirrors.pingcap.com/5354.tiflash.json failed: unexpected EOF
```