vcpkg installation script requires Linux 64-bit

[jjcnn]

The installation instructions and the vcpkg.sh script assumes the user is on a Linux 64-bit architecture. This PR changes the script to make an explicit check to ensure the assumption is satisfied.

Further, INSTALL.md has been updated to reflect the changes.

@ilyasergey : I think you're the only one in the Scilla team who has access to a Mac. Could you check that this works/doesn't work as expected?

[anton-trunov]

Hey @jjcnn, I cannot clone the repo anymore:

❯ git clone --jobs 4 --recurse-submodules https://github.com/Zilliqa/scilla
Cloning into 'scilla'...
remote: Enumerating objects: 30519, done.
remote: Counting objects: 100% (907/907), done.
remote: Compressing objects: 100% (554/554), done.
remote: Total 30519 (delta 494), reused 677 (delta 345), pack-reused 29612
Receiving objects: 100% (30519/30519), 12.10 MiB | 463.00 KiB/s, done.
Resolving deltas: 100% (23660/23660), done.
Submodule 'deps/cryptoutils/src' (https://github.com/Zilliqa/cryptoutils) registered for path 'deps/cryptoutils/src'
Submodule 'deps/schnorr/src' (https://github.com/Zilliqa/schnorr) registered for path 'deps/schnorr/src'
Cloning into '/Users/anton/zilliqa/scilla/deps/cryptoutils/src'...
remote: Enumerating objects: 147, done.
remote: Counting objects: 100% (147/147), done.
remote: Compressing objects: 100% (105/105), done.
remote: Total 147 (delta 51), reused 116 (delta 29), pack-reused 0
Receiving objects: 100% (147/147), 79.73 KiB | 208.00 KiB/s, done.
Resolving deltas: 100% (51/51), done.
Cloning into '/Users/anton/zilliqa/scilla/deps/schnorr/src'...
remote: Enumerating objects: 341, done.
remote: Counting objects: 100% (82/82), done.
remote: Compressing objects: 100% (30/30), done.
remote: Total 341 (delta 56), reused 54 (delta 52), pack-reused 259
Receiving objects: 100% (341/341), 106.51 KiB | 200.00 KiB/s, done.
Resolving deltas: 100% (193/193), done.
fatal: remote error: upload-pack: not our ref 13a9728da8469420fac9cc1638c5001ee7d58827
fatal: Fetched in submodule path 'deps/cryptoutils/src', but it did not contain 13a9728da8469420fac9cc1638c5001ee7d58827. Direct fetching of that commit failed.