Invalid cross build feature flag (#3959)

## Issue Addressed

The documentation referring to build from source mismatches with the what gitworkflow uses. 

aa5b7ef783/book/src/installation-source.md (L118-L120)

## Proposed Changes

Because the github workflow uses `cross` to build from source and for that build there is different env variable `CROSS_FEATURES` so need pass at the compile time. 

## Additional Info

Verified that existing `-dev` builds does not contains the `minimal` spec enabled. 

```bash
> docker run --rm --name node-5-cl-lighthouse sigp/lighthouse:latest-amd64-unstable-dev lighthouse --version
Lighthouse v3.4.0-aa5b7ef
BLS library: blst-portable
SHA256 hardware acceleration: true
Allocator: jemalloc
Specs: mainnet (true), minimal (false), gnosis (true)
```

.github/workflows/docker.yml

@@ -52,8 +52,8 @@ jobs:
                          x86_64,
                          x86_64-portable]
                 features: [
-                    {version_suffix: "", env: ""},
-                    {version_suffix: "-dev", env: "spec-minimal"}
+                    {version_suffix: "", env: "gnosis,slasher-lmdb,slasher-mdbx,jemalloc"},
+                    {version_suffix: "-dev", env: "gnosis,slasher-lmdb,slasher-mdbx,jemalloc,spec-minimal"}
                 ]
                 include:
                     - profile: maxperf
@@ -66,6 +66,7 @@ jobs:
             VERSION_SUFFIX: ${{ needs.extract-version.outputs.VERSION_SUFFIX }}
             FEATURE_SUFFIX: ${{ matrix.features.version_suffix }}
             FEATURES: ${{ matrix.features.env }}
+            CROSS_FEATURES: ${{ matrix.features.env }}
         steps:
             - uses: actions/checkout@v3
             - name: Update Rust
@@ -76,7 +77,7 @@ jobs:
             - name: Cross build Lighthouse binary
               run: |
                   cargo install cross
-                  env CROSS_PROFILE=${{ matrix.profile }} make build-${{ matrix.binary }}
+                  env CROSS_PROFILE=${{ matrix.profile }} CROSS_FEATURES=${{ matrix.features.env }} make build-${{ matrix.binary }}
             - name: Move cross-built binary into Docker scope (if ARM)
               if: startsWith(matrix.binary, 'aarch64')
               run: |