Add cache ExtLibraries in build CI

[sksat]

Overview

Speed-up build CI by cache ExtLibraries.

Issue

N/A

Details

• change ExtLibraries install directory into ${GITHUB_WORKSPACE}/ExtLibraries (external deps install dir)
• change checkout directory into ${GITHUB_WORKSPACE}/s2e-core
• cache ${GITHUB_WORKSPACE}/ExtLibraries

Validation results

• without cache: https://github.com/ut-issl/s2e-core/runs/4935695447 (total 7m54s)
• cache hit: https://github.com/ut-issl/s2e-core/actions/runs/1745010060 (total 4m29s)

Scope of influence

eg. The behavior of XX will be change.

Supplement

Write additional comments if you need.

Note

• If there are related Projects, tie them together.
• Assignees should be set if possible.
• Reviewers should be set if possible.
• Set priority label if possible.

[meltingrabbit]

良さそう．

そういえば，これってキャッシュが使えるか否か（新しくまたcurlしなくていいかどうか）ってどこで判断してるの？

[meltingrabbit]

上記納得したので，LGTMです．

[sksat]

ありがとうございます．マージします．