Install fails when setting URL with context path to G_MIRROR

[tkyggit]

Describe the bug/缺陷描述 Install fails when setting URL with context path to G_MIRROR

To Reproduce/缺陷复现步骤 Steps to reproduce the behavior:

1. set G_MIRROR with context path url, like https://aaa.com/bbb/dl. context means bbb
2. g ls-remote is fine
3. g install 1.21.0
4. Trying to download from wrong URL　[g] Url "https://aaa.com/dl/go1.21.0.windows-amd64.zip" is unreachable ==> 404

Expected behavior/期望行为 expected download url is 'https://aaa.com/bbb/dl', but 'https://aaa.com/dl' missing context path bbb

Screenshots/实际运行结果截图 If applicable, add screenshots to help explain your problem.

Environments (please complete the following information)/运行环境:

• OS[e.g. uname -a] Windows 10

• g -v

1.6.0 ```

• go env

• echo $G_EXPERIMENTAL

• echo $G_HOME

• echo $G_MIRROR

　　https://aaa.com/bbb/dl ```

Additional context/额外信息 Add any other context about the problem here.

[voidint]

Unable to reproduce the issue. Please follow the steps below:

• Set the G_MIRROR environment variable(export G_MIRROR=https://aaa.com/bbb/dl)
• Execute command: g env
• Execute command: g install 1.21.0

Please provide the complete output of the above commands.

[voidint]

https://github.com/voidint/g/blob/master/collector/official/official_collector.go#L58