Set $TARGET_FLAGS and $TARGET_DIR from matrix

Since they are produced via `${{ }}` interpolation using values
from the matrix.

Matrix values cannot be formed from job-level `env`, but job-level
`env` can be formed form matrix values, as this does.

Author: EliahKagan

.github/workflows/release.yml

@@ -119,9 +119,9 @@ jobs:
       # For some builds, we use cross to test on 32-bit, and maybe later big-endian, systems.
       CARGO: cargo
       # Whether CARGO is `cargo` or `cross`, TARGET_FLAGS will be set to `--target matrix.target`.
-      TARGET_FLAGS: ''
+      TARGET_FLAGS: --target=${{ matrix.target }}
       # Whether CARGO is `cargo` or `cross`, TARGET_DIR will include matrix.target.
-      TARGET_DIR: ./target
+      TARGET_DIR: ./target/${{ matrix.target }}
       # Emit backtraces on panics.
       RUST_BACKTRACE: 1
 
@@ -148,11 +148,6 @@ jobs:
           cargo install cross
           echo 'CARGO=cross' >> "$GITHUB_ENV"
 
-      - name: Set target variables
-        run: |
-          echo 'TARGET_FLAGS=--target ${{ matrix.target }}' >> "$GITHUB_ENV"
-          echo 'TARGET_DIR=./target/${{ matrix.target }}' >> "$GITHUB_ENV"
-
       - name: Show command used for Cargo
         run: |
           echo 'cargo command is: ${{ env.CARGO }}'