gnu: Add embree-3.

Fixes <https://issues.guix.gnu.org/73186>.  Followup to commit
31a55a652f which updated embree; libigl needs an
older version.

* gnu/packages/engineering.scm (embree-3): New variable.
* gnu/packages/engineering.scm (libigl)[inputs]: Replace embree with embree-3

Change-Id: I8c1f038b918c8f4411fec7f53703945df6e6008a

gnu/packages/engineering.scm

@@ -3845,7 +3845,7 @@ (define (fix-external-library cmake source)
      `(("boost" ,boost)
        ("cgal" ,cgal)
        ("eigen" ,eigen)
-       ("embree" ,embree)
+       ("embree" ,embree-3)
        ("glfw" ,glfw-3.4)
        ("gmp" ,gmp)
        ("mesa" ,mesa)

gnu/packages/graphics.scm

@@ -475,6 +475,21 @@ (define-public embree
 applications.")
     (license license:asl2.0)))
 
+(define-public embree-3
+  (package
+    (inherit embree)
+    (name "embree")
+    (version "3.3.15")
+    (source (origin
+              (method git-fetch)
+              (uri (git-reference
+                    (url "https://github.com/embree/embree")
+                    (commit (string-append "v" version))))
+              (file-name (git-file-name name version))
+              (sha256
+               (base32
+                "1kcvz7g6j56anv9zjyd3gidxl46vipw0gg82lns12m45cd43iwxm"))))))
+
 (define-public openvdb
   (package
     (name "openvdb")