build: cargo-build-system: Add pkg-config environment variables.

* guix/build/cargo-build-system.scm (configure): Add environment
variables to use system libgit2 and libssh2.
* gnu/packages/crates-io.scm (rust-libgit2-sys-0.10,
rust-libssh2-sys-0.2)[arguments]: Remove LIBGIT2 and LIBSSH2 environment
variable settings.
* gnu/packages/rust-apps.scm (tokei)[arguments]: Same.

gnu/packages/crates-io.scm

@@ -5626,8 +5626,6 @@ values of all the exported APIs match the platform that libc is compiled for.")
            (lambda* (#:key inputs #:allow-other-keys)
              (let ((openssl (assoc-ref inputs "openssl")))
                (setenv "OPENSSL_DIR" openssl))
-             (setenv "LIBGIT2_SYS_USE_PKG_CONFIG" "1")
-             (setenv "LIBSSH2_SYS_USE_PKG_CONFIG" "1")
              #t)))))
     (native-inputs
      `(("libgit2" ,libgit2)
@@ -5734,7 +5732,6 @@ functions and static variables these libraries contain.")
            (lambda* (#:key inputs #:allow-other-keys)
              (let ((openssl (assoc-ref inputs "openssl")))
                (setenv "OPENSSL_DIR" openssl))
-             (setenv "LIBSSH2_SYS_USE_PKG_CONFIG" "1")
              #t)))))
     (native-inputs
      `(("libssh2" ,libssh2)

gnu/packages/rust-apps.scm

@@ -143,9 +143,7 @@ gitignore rules.")
          (add-after 'configure 'unvendor-libraries-from-crates
            (lambda* (#:key inputs #:allow-other-keys)
              (let ((openssl (assoc-ref inputs "openssl")))
-               (setenv "OPENSSL_DIR" openssl)
-               (setenv "LIBGIT2_SYS_USE_PKG_CONFIG" "1")
-               (setenv "LIBSSH2_SYS_USE_PKG_CONFIG" "1"))
+               (setenv "OPENSSL_DIR" openssl))
              #t)))))
     (native-inputs
      `(("libgit2" ,libgit2)

guix/build/cargo-build-system.scm

@@ -119,6 +119,8 @@ directory = '" port)
   ;; upgrading the compiler for example.
   (setenv "RUSTFLAGS" "--cap-lints allow")
   (setenv "CC" (string-append (assoc-ref inputs "gcc") "/bin/gcc"))
+  (setenv "LIBGIT2_SYS_USE_PKG_CONFIG" "1")
+  (setenv "LIBSSH2_SYS_USE_PKG_CONFIG" "1")
 
   ;; We don't use the Cargo.lock file to determine the package versions we use
   ;; during building, and in any case if one is not present it is created