Updates plrust/build to programmatically get pgrx version (#381)

Addresses: #377

When plrust goes through a release cycle, occasionally (read: pretty
much "almost" every time) we forget to update the hard-coded pgrx
version in the `plrust/build` script due to the fact that our automated
release tooling does not consider nor modify the script file. With the
changes in this PR, we programmatically get the pgrx version using
`cargo tree` and some unix utils magic so that nobody has to think about
updating that value ever again upon release.

Author: BradyBonnette

plrust/build

@@ -52,9 +52,18 @@ fi
 cargo update -p pgrx
 cargo fetch
 if [ "$CI" != true ]; then
-    cargo install cargo-pgrx \
-     --version "0.10.0" \
-     --locked # use the Cargo.lock in the pgrx repo
+    # Attempt to get pgrx version from cargo tree. Looking for a pattern such as "pgrx v0.10.0"
+    PGRX_VERSION=$(cargo tree --depth 1 --package plrust | grep -E "\s*pgrx\s+v[0-9]+\.[0-9]+\.[0-9]+" | head -n 1 | cut -f2- -dv)
+
+    if [ -z "$PGRX_VERSION" ]; then
+        echo "Could not determine pgrx version from 'cargo tree'!"
+        exit 1
+    else
+        echo "Installing cargo-pgrx version $PGRX_VERSION"
+        cargo install cargo-pgrx \
+            --version "$PGRX_VERSION" \
+            --locked # use the Cargo.lock in the pgrx repo
+    fi
 fi
 
 # Don't need to run cargo pgrx init: user might already have set it up,