fix(devops): Ask about clobbering

[bitdivine]

Motivation

When the signer wasm is updated, the existing code would not download the new signer.

Changes

• If there is an existing signer candid or Wasm, ask the caller whether they would like to replace it.

Tests

When there are no existing files, they are downloaded:

max@sinkpad:~/dfn/oisy-wallet/branches/fix-deploy-frontend (15:34)$ rm target/signer.*
max@sinkpad:~/dfn/oisy-wallet/branches/fix-deploy-frontend (16:40)$ scripts/build.signer.sh
Downloading https://github.com/dfinity/chain-fusion-signer/releases/download/v0.2.3/signer.did --> target/signer.did
Downloading https://github.com/dfinity/chain-fusion-signer/releases/download/v0.2.3/signer.wasm.gz --> target/signer.wasm.gz

With existing files, No is the default. "n" "N" and "no" are recognized.

Overwrite existing target/signer.did? [y/N] n
Using existing signer candid file.

Overwrite existing target/signer.wasm.gz? [y/N] 
Using existing signer wasm file.

Overwrite existing target/signer.did? [y/N] N
Using existing signer candid file.

Overwrite existing target/signer.wasm.gz? [y/N] no
Using existing signer wasm file.

With existing files, "y" "Y" "yes" and "YES!!!" allows the files to be overwritten:

Overwrite existing target/signer.did? [y/N] y
Downloading https://github.com/dfinity/chain-fusion-signer/releases/download/v0.2.3/signer.did --> target/signer.did

Overwrite existing target/signer.wasm.gz? [y/N] Y
Downloading https://github.com/dfinity/chain-fusion-signer/releases/download/v0.2.3/signer.wasm.gz --> target/signer.wasm.gz

Overwrite existing target/signer.did? [y/N] yes
Downloading https://github.com/dfinity/chain-fusion-signer/releases/download/v0.2.3/signer.did --> target/signer.did

Overwrite existing target/signer.wasm.gz? [y/N] YES!!!
Downloading https://github.com/dfinity/chain-fusion-signer/releases/download/v0.2.3/signer.wasm.gz --> target/signer.wasm.gz