Fix numbering of steps during build

[SkymanOne]

The solution simply bumps the number of steps in the build workflow.

The result for build with all flag:

cargo contract build --skip-linting --generate all
 [1/6] Skip ink! linting rules
 [2/6] Building cargo project
    Updating crates.io index
    Finished release [optimized] target(s) in 0.26s
 [3/6] Post processing wasm file
 [4/6] Optimizing wasm file
 [5/6] Generating metadata
    Updating crates.io index
   Compiling metadata-gen v0.1.0 (/private/var/folders/4v/n3610fp154z5qxgdqf8nnpj80000gn/T/cargo-contract_dxoOs4/.ink/metadata_gen)
    Finished release [optimized] target(s) in 0.79s
     Running `target/ink/release/metadata-gen '' `
 [6/6] Generating bundle
...

The result for build with code-only flag:

cargo contract build --skip-linting --generate code-only
 [1/4] Skip ink! linting rules
 [2/4] Building cargo project
    Updating crates.io index
    Finished release [optimized] target(s) in 1.36s
 [3/4] Post processing wasm file
 [4/4] Optimizing wasm file

Original wasm size: 47.5K, Optimized: 22.0K

The contract was built in DEBUG mode.

Your contract's code is ready. You can find it here:
...

[cla-bot-2021]

User @SkymanOne, please sign the CLA here.

[SkymanOne]

Resolves #592

[HCastano]

Thanks!

[cmichi]

Thank you!

No biggie for this, but as a sidenote: Generally it's good to already make an entry for user facing changes in the [Unreleased] section of https://github.com/paritytech/cargo-contract/blob/master/CHANGELOG.md together with the PR. Then we don't overlook things for the next release.