.cargo/config.toml for specifying linker

To avoid code duplication we specify the linker for the specific
target armv7-unknown-linux-gnueabihf in the config.toml file as
specified here:
https://doc.rust-lang.org/cargo/reference/config.html#targettriplelinker

.cargo/config.toml

@@ -0,0 +1,2 @@
+[target.armv7-unknown-linux-gnueabihf]
+linker = "arm-linux-gnueabihf-gcc"

.github/workflows/ci.yml

@@ -64,8 +64,6 @@ jobs:
           sudo apt-get install gcc-arm-linux-gnueabihf
 
       - name: Build binary
-        env:
-          CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_LINKER: arm-linux-gnueabihf-gcc
         run: |
           cargo build -p swap --target ${{ matrix.target }}
 
@@ -100,8 +98,6 @@ jobs:
       - uses: Swatinem/rust-cache@v1.2.0
 
       - name: Build tests
-        env:
-          CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_LINKER: arm-linux-gnueabihf-gcc
         run: cargo build --tests --workspace --all-features
 
       - name: Run monero-harness tests

.github/workflows/release-cli.yml

@@ -55,8 +55,7 @@ jobs:
           sudo apt-get install gcc-arm-linux-gnueabihf
 
       - name: Build ${{ matrix.target }} ${{ matrix.bin }} release binary
-        env:
-          CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_LINKER: arm-linux-gnueabihf-gcc
+
         run: cargo build --target=${{ matrix.target }} --release --package swap --bin ${{ matrix.bin }}
 
       - name: Smoke test the binary