Update actions/cache action to v2.1.6

Author: renovate-bot

.github/workflows/ci.yml

@@ -77,7 +77,7 @@ jobs:
 
       # Current size as of 2021-02-15: ~105 MB
       - name: Cache cargo registry and git deps
-        uses: actions/[email protected].4
+        uses: actions/[email protected].6
         with:
           path: |
             ~/.cargo/registry/cache
@@ -92,7 +92,7 @@ jobs:
           echo "::set-output name=version::$(rustc -V)"
 
       - name: Cache cargo build
-        uses: actions/[email protected].4
+        uses: actions/[email protected].6
         with:
           path: target
           key: v2-${{ runner.os }}-cargo-clippy-${{ steps.rustc.outputs.version }}-${{ hashFiles('**/Cargo.lock') }}
@@ -167,14 +167,14 @@ jobs:
       #
       # Current size as of 2021-02-15: ~15 MB
       - name: Cache cargo binaries
-        uses: actions/[email protected].4
+        uses: actions/[email protected].6
         with:
           path: ~/.cargo/bin
           key: ${{ runner.os }}-cargo-bin-${{ matrix.rust }}-${{ hashFiles('.diesel_version') }}-v3
 
       # Current size as of 2021-02-15: ~105 MB
       - name: Cache cargo registry and git deps
-        uses: actions/[email protected].4
+        uses: actions/[email protected].6
         with:
           path: |
             ~/.cargo/registry/cache
@@ -200,7 +200,7 @@ jobs:
 
       # Current size as of 2021-02-12: ~325 MB
       - name: Cache cargo build
-        uses: actions/[email protected].4
+        uses: actions/[email protected].6
         with:
           path: target
           key: v2-${{ runner.os }}-cargo-build-target-${{ steps.rustc.outputs.version }}-${{ hashFiles('**/Cargo.lock') }}